Document main-relative companion loading

docs/adr/0022-main-relative-context.md

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+# ADR 0022: Bind Companion Contexts After Main Selection
+
+## Status
+
+Accepted.
+
+## Date
+
+2026-11-09
+
+## Context
+
+Applications choose a main-file search and may also declare companion imports
+and a selected overlay. The main directory is only known after discovery, so
+supplying every dependent context before loading duplicates selection or requires
+a preliminary load. ADR 0019 keeps main discovery explicit and inside loading.
+
+## Decision
+
+Extend layered loading to bind application-declared non-file underlays to the
+resolved main context after acquisition. An explicit underlay context overrides
+the binding, and an explicit fallback or require-base option controls absence.
+Selected layered overlays choose an explicit context, the resolved main context,
+or a fallback in that order. Derived and fallback lookup use the context base;
+the inherited root continues to bound containment. Ordinary and explicit
+overlay lookup retain their established root-relative behavior.
+
+Applications remain responsible for choosing searches, import declarations,
+and overlay names. Selection failures are terminal; no second search or
+application context callback is introduced.
+
+## Alternatives Considered
+
+- Require applications to discover the main file separately. This repeats
+  discovery and may run a complete load more than once.
+- Implicitly scan conventional companion directories. This would impose
+  application-specific conventions without an explicit declaration.
+- Add only a relative file constructor. This would not cover mapping or bytes
+  sources with existing import, placement, and composition rules.
+
+## Consequences
+
+The load owns the resolved binding and keeps namespace, containment, source
+identity, and handle lifetime in the existing acquisition lifecycle. A missing
+base skips optional imports while preserving the declaring payload; required
+imports and require-base retain explicit failures. The configuration contract
+and public declarations describe the observable rules.