Write release notes for v0.19.0

docs/releases/v0.19.0.md

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+# Promptkit v0.19.0
+
+This release completes two rounds of audit remediation, correcting error and
+diagnostic reporting and simplifying configuration, validation, and value
+ownership. It includes intentional compatibility changes for custom artifact
+readers, programmatic configuration sources, and schemas. The minimum supported
+Go version remains 1.27.0; ConfigKit remains at v1.1.0.
+
+## Compatibility And Upgrade
+
+Consumers upgrading from v0.18.0 should review these changes:
+
+- A successful `ArtifactReader.Read` transfers ownership of the returned
+  artifact and its mutable body storage to Promptkit. Custom readers must stop
+  mutating returned values through retained aliases. Readers that reuse scratch
+  buffers must return independent storage. Promptkit retains its defensive
+  body copy, but that copy does not make concurrent buffer reuse safe. See the
+  [extension guidance](../consumers/pkg-promptkit.md#extension-interfaces).
+- Provider maps passed through `ProfileSource` and `BackendSource` now use
+  ConfigKit's native Mapping input forms directly: built-in scalars, `[]any`,
+  and `map[string]any` recursively. Convert `json.Number` and typed containers
+  explicitly to supported values, choosing numeric precision appropriate to
+  the application. The former recursive compatibility conversion is removed.
+  Runtime execution overrides retain their separate accepted-value behavior.
+- Schemas used for local validation and native structured output must be
+  self-contained. Move externally referenced definitions into the root document
+  and use internal references or explicit schema IDs. References requiring
+  another file or network resource, or relying on the source filename as a base,
+  fail during preparation. Promptkit does not bundle external schemas. See the
+  [schema reference](../formats.md#schemas).
+
+Prepared schemas no longer inherit the depth and node limits used when copying
+runtime provider values. Schemas previously rejected solely by those incidental
+limits may now be accepted; decoder/compiler checks and the self-contained
+schema requirement still apply.
+
+No configuration-format migration is required from v0.18.0. After addressing
+any affected integrations:
+
+```sh
+go get go.maximumdirect.net/promptkit@v0.19.0
+```
+
+## Errors And Diagnostics
+
+Public errors and terminal observations now preserve the category of the
+operation that failed even when an injected collaborator returns an error from
+another Promptkit operation. Underlying causes remain available through
+`errors.Is` and `errors.As`, without repeatedly duplicating nested cause trees.
+
+Built-in generation preserves response-body read failures separately from
+malformed response content, and failures during request preparation are
+distinguished from dispatched attempts.
+
+OpenRouter authentication diagnostics bound response recognition to 1 MiB.
+An oversized HTTP 200 response reports
+`SmokeTestReasonAuthenticationResponseLimitExceeded`, leaving credential
+acceptance unknown. Cancellation during the read remains an interruption.
+Consumers that enumerate diagnostic reasons should handle the new reason;
+see the [diagnostic integration contract](../integrations/openai-compatible-chat.md#explicit-smoke-diagnostics).
+
+## Internal Simplification And Ownership
+
+Prompt lookup and rendering now consistently use captured configuration, and
+smoke syntax and schema checks share the prepared output-validation lifecycle.
+Unused routing defaults, alternate private workflows, and duplicate wire
+representations have been removed.
+
+Provider-setting resolution, inspection, prepared details, and smoke reports
+avoid redundant validation and copying of already accepted values. Prepared
+execution shares one private redacted metadata snapshot while preserving
+independent caller-visible values. Provider values are detached before custom
+JSON encoders run, protecting retained metadata from encoder-side mutation.
+
+Requirement analysis, namespace-filtered listings, and repair-feedback
+construction also avoid unnecessary traversal and temporary allocation. These
+changes preserve existing request isolation and execution behavior without
+introducing new performance guarantees.
+
+[COPYRIGHT](https://forge.maximumdirect.net/base-layer/promptkit/src/tag/v0.19.0/COPYRIGHT) | [LICENSE](https://forge.maximumdirect.net/base-layer/promptkit/src/tag/v0.19.0/LICENSE) | [ATTRIBUTION](https://forge.maximumdirect.net/base-layer/promptkit/src/tag/v0.19.0/docs/oss/attribution.md)