Settle main-source eligibility for relative context binding

docs/roadmap/main-relative-inputs.md

@@ -1,8 +1,7 @@
 # Main-relative companion imports and overlays
 
-Status: accepted direction, public API structure, absent-main behavior, and
+Status: all four design decisions settled; not implemented. Final API spelling
-containment/symlink convention; main-source eligibility remains open.
+and implementation planning remain. Baseline: `191015e`.
-Not implemented. Baseline: `191015e`.
 
 ## Purpose
 
@@ -14,7 +13,8 @@ boilerplate to the main file or creating a manifest on disk.
 
 The central addition is deferred filesystem-context binding for application-owned
 underlay inputs, including `Mapping` and `Bytes` sources containing native import
-declarations. Bind their imports to the selected main directory during loading.
+declarations. Bind their imports to the main input's resolved filesystem context
+during loading: for file mains, lookup starts beside the selected main file.
 Reuse existing optional-file, directory, recursion, pattern, destination, and
 composition behavior. A new file constructor alone would not cover this workflow,
 and a second fragment-discovery implementation is unnecessary.
@@ -32,6 +32,11 @@ root override, `/srv/campaign/audita.yml` supplies the location
 lookup uses the configuration root, which can differ from the main directory.
 Optional discovery exhaustion provides no inferred context.
 
+Non-file mains can already carry an explicit filesystem context. Absent inputs,
+including optional discovery exhaustion, can also retain a supplied context;
+for an absent host input, an explicit root supplies the base when `BaseDir` is
+omitted. Reusing these validated contexts requires no filename inference.
+
 The [layered API](../config.md#layered-loading) has the coordination gap:
 
 - An active overlay requires `LayeredOptions.OverlayFilesystem`. Preflight
@@ -94,10 +99,11 @@ selects main-derived filesystem binding for that input. `Mapping` and `Bytes`
 are first-class supported sources for this use. OD-1 settles the public API
 structure below; no new operator-facing YAML syntax is needed.
 
-Once the main is selected, ConfigKit supplies the underlay import base from
+Once the main context is resolved, ConfigKit inherits its base, namespace, and
-that file's resolved directory and inherits its namespace and containment root
+containment root under OD-3 and OD-4. File mains supply their resolved directory;
-under OD-3. Existing import selection then acquires the companion files. Each
+non-file or absent mains may supply an explicit context. Existing import selection
-imported file uses its own directory for further relative imports. An import of
+then acquires the companion files. Each imported file uses its own directory for
+further relative imports. An import of
 `extra.yml` from `glossary/terms.yml` resolves to `glossary/extra.yml`.
 
 Applications choose underlay groups and their order. A group composes strictly;
@@ -118,14 +124,15 @@ native imports. Ordinary `File` paths do not silently gain new relative semantic
 ### Layered overlays
 
 For an active layered overlay, use an explicit `OverlayFilesystem` when supplied.
-Otherwise derive the overlay context automatically from the available main-file
+Otherwise derive the overlay context automatically from the main input's usable
-information; the common workflow requires no `OverlayFromMain` flag. An invalid
+resolved context; the common workflow requires no `OverlayFromMain` flag. An
-explicit context fails rather than falling back. Inactive overlays need no
+invalid explicit context fails rather than falling back. Inactive overlays need no
 context. Missing-main and fallback cases are addressed below and in OD-2.
 
-For this newly derived layered context, the lookup location is the main file's
+For this newly derived layered context, lookup uses the main context's resolved
-directory, independently of a wider containment root. If the main is
+base, independently of a wider containment root. For file mains this is the
-`/srv/campaign/audita.yml` and its permitted root is `/srv`, the selected file
+main file's directory. If the main is `/srv/campaign/audita.yml` and its permitted
+root is `/srv`, the selected file
 is `/srv/campaign/overlays/production.yml`, not `/srv/overlays/production.yml`.
 Widening containment must not relocate the automatically selected companions.
 
@@ -138,7 +145,8 @@ precedence, and validation retain their existing semantics.
 ### Absent main and optional companions
 
 Optional discovery may find no main file while defaults and CLI settings still
-permit a successful application load. When no usable base is available, skip
+permit a successful application load. An explicit context on that absent main
+can still supply a usable base under OD-4. When no usable base is available, skip
 optional companion import expansion while retaining the declaring input's
 ordinary payload and validation. Required imports still fail. Applications can
 require a base or supply an explicit fallback context on each derived binding.
@@ -188,21 +196,22 @@ No release version or additional release gate is selected here.
 
 ## Work from the baseline
 
-1. **Finalize context declarations and absence policy.** Resolve the remaining
+1. **Finalize API spelling and examples.** Translate the settled decisions below
-   choices below and prepare Go examples for Audita's mapping/bytes workflow,
+   into concrete declarations and prepare Go examples for Audita's mapping/bytes
-   explicit context overrides, and independently located embedded inputs.
+   workflow, explicit overrides, non-file main contexts, and independently located
+   embedded inputs.
 2. **Carry binding intent through the public facade and load request.** Update
    the relevant input/options declarations and conversions in `layered.go` and
    `lifecycle.go`, retaining root-owned public types and normal ownership rules.
    Distinguish a deferred relationship from a concrete filesystem context.
-3. **Bind underlay import contexts after main selection.** Extend preparation
+3. **Bind underlay import contexts after main context resolution.** Extend
-   and acquisition in `internal/engine/load_request.go`, `load_input.go`, and
+   preparation and acquisition in `internal/engine/load_request.go`, `load_input.go`, and
-   `acquisition.go` as needed. Reuse the acquired main's directory and namespace,
+   `acquisition.go` as needed. Reuse the main's resolved base, root, and namespace,
-   apply the chosen missing-base policy, and delegate selection to existing import
+   apply the settled missing-base policy, and delegate selection to existing import
    machinery. Preserve independently supplied contexts.
 4. **Derive overlay context and separate lookup from containment.** Remove the
    unconditional explicit-context requirement for the new layered default.
-   Preserve explicit and ordinary lookup while enabling a main-directory lookup
+   Preserve explicit and ordinary lookup while using the main context's resolved
    base inside a potentially wider root. Reuse selected-overlay loading and its
    failure and resource-ownership behavior.
 5. **Verify the complete workflow and document it.** Add maintained consumer
@@ -219,6 +228,9 @@ Validate behavior through the public loading boundary, including:
 
 - Explicit and discovered main files, early and later search candidates, and an
   unrelated invocation directory. Companions follow the same selected main.
+- Native file mains, bytes/readers/mappings with explicit host or native contexts,
+  and absent mains with explicit contexts. Cover absent host root-only contexts,
+  missing and invalid contexts, and context reuse without enabling main imports.
 - Mapping and bytes underlays containing optional named-file and recursive
   directory imports with `*.yml`/`*.yaml` filtering and mounted destinations.
 - File only, directory only, both, neither, empty directories, and invalid or
@@ -308,6 +320,9 @@ context in this order:
 
 A fallback can satisfy require-base: the option requires a usable context, not
 an actual main file. Consumers requiring a main file can make discovery required.
+Under OD-4, an explicit context on a non-file or absent main can also satisfy it;
+the absence policy applies when no usable base exists, not merely when no main
+content exists.
 An invalid selected context fails; it does not activate a lower-priority choice.
 Fallback applies only when the main cannot provide a base, never after a failure
 reading or processing a selected main, or after missing or invalid companion
@@ -326,10 +341,11 @@ Companion absence settings do not make a selected overlay optional.
 
 ### OD-3: Containment inheritance and symlink convention — settled
 
-Derived companion imports and derived layered overlays inherit the selected
+Derived companion imports and derived layered overlays inherit the main input's
-main's resolved filesystem namespace and containment root. Lookup starts at the
+resolved filesystem namespace and containment root. Lookup uses its resolved
-main file's resolved directory, even when its configured root is wider. Keep
+base under OD-4, which for file mains is the file's resolved directory, even when
-existing explicit overlay context and ordinary `Load` lookup behavior.
+its configured root is wider. Keep existing explicit overlay context and ordinary
+`Load` lookup behavior.
 
 For a host main at `/srv/campaign/audita.yml` with root `/srv`, companion
 `glossary.yml` resolves to `/srv/campaign/glossary.yml`, while an import of
@@ -352,27 +368,41 @@ under the normal rules when they choose another location or scope.
 Each imported file continues to resolve further imports from its own directory,
 within the inherited root. Existing import `follow` controls still govern
 discovered symlink entries; they do not select the base of a symlinked main.
-Preserve existing containment checks and traversal controls. For any eligible
+Preserve existing containment checks and traversal controls. For native main
-native main under OD-4, retain that filesystem's namespace and link-handling
+contexts under OD-4, retain that filesystem's namespace and link-handling
 capabilities without imposing host-filesystem guarantees on every `fs.FS`.
 
-## Open Decisions
+### OD-4: Main-source eligibility beyond discovered host files — settled
 
-### OD-4: Main-source eligibility beyond discovered host files
+Derive from any usable, validated main filesystem context, regardless of the
+main source's format. Document `MainRelative` as using the main input's resolved
+filesystem context. For the common file workflow, this means lookup beside the
+selected main file. Reuse existing context resolution rather than adding another
+acquisition mechanism or requiring main imports to be enabled.
 
-**Settled:** both explicitly selected and discovered host mains support the
+- Explicit host files and successful discovery supply the canonical file directory
-workflow. Non-file companion inputs, especially mappings and bytes, are in scope;
+  and resolved containment root.
-this decision concerns the main input that supplies their base.
+- `Filesystem.File` mains supply their directory and root within their existing
+  namespace. Dependent loading stays in that namespace.
+- `Bytes`, `Reader`, and `Mapping` mains can supply an explicit context with a
+  usable base, root, and namespace under their existing validation rules.
+- `NoSource` and optional discovery exhaustion can retain an explicit main
+  context. Reuse its resolved base: existing absent-host rules allow an explicit
+  root to serve as the base when `BaseDir` is omitted; native contexts retain
+  their existing requirements.
 
-**Recommendation:** also support native `Filesystem.File` mains using their
+For example, a bytes main with host `BaseDir: /srv/campaign` and `Root: /srv`
-namespace's path and ownership rules. For an initial file-based scope, non-file
+can supply the same companion lookup and containment as a file main in that
-mains use independently supplied or fallback contexts rather than implicitly
+directory with the same root. An absent main with a usable explicit context
-providing a main-file directory. This covers the requested workflow and avoids
+can likewise supply companion and overlay lookup without creating main content.
-making discovery a prerequisite for equivalent file inputs.
 
-**Alternative:** derive from any usable main context, including `BaseDir` on a
+Without a usable main context, apply OD-2's fallback, optional-import skipping,
-byte, reader, or mapping main. This is more uniform with context-based loading
+or error behavior. Invalid supplied contexts fail rather than selecting a
-and avoids excluding a known base, but requires explaining main-relative as
+fallback. Never infer filesystem locations from diagnostic labels or unsuccessful
-main-context-relative and deciding how root-only or absent-main contexts behave.
+search candidates, manufacture a main source, or change source evidence to make
-A host-only initial scope is smaller but leaves native-file consumers with the
+a non-file input appear file-backed.
-same coordination gap.
+
+This preserves one context-reuse rule when applications change how they supply
+main content. OD-1's concrete overrides and OD-3's containment, namespace, and
+link-handling rules continue to apply. Existing ordinary `Load` and explicit
+overlay context lookup semantics remain unchanged.