docs(xtask): generate and validate crate support matrix from workspace metadata

[EffortlessSteven]

Motivation

• Provide a single source-of-truth crate-by-crate support contract so consumers know which crates are intended for public use vs internal/test-only adapters.
• Automate generation and validation so the matrix cannot drift from workspace state and CI will catch inconsistencies.

Description

• Add a support_matrix section to docs/metadata/workspace-docs.json describing per-crate fields: support_tier, publish_status, facade_exposed, semver_expectation, msrv_policy, intended_audience, and optional replacement_path / deprecation_note.
• Extend xtask docs sync to render and validate the support matrix: xtask/src/docs_sync.rs now reads support_matrix, validates enum values and illegal combinations (e.g. test-only must be repo-internal, facade_exposed incompatible with repo-internal), ensures completeness against the Cargo workspace, and writes docs/reference/support-matrix.md (generated from metadata).
• Integrate the docs-sync step into CI and gate flows by invoking docs_sync::docs_sync_cmd(true) in xtask/src/main.rs so metadata drift fails automation.
• Add unit tests for rendering/consistency and validation in xtask/src/docs_sync.rs and add the generated docs/reference/support-matrix.md file; update README.md and docs/how-to/release.md to reference the support matrix.

Testing

• Ran cargo xtask docs-sync to generate docs/reference/support-matrix.md and confirmed it wrote the file successfully. (Succeeded.)
• Ran cargo xtask docs-sync --check to verify no drift between metadata and generated docs; command returned success in CI-like check. (Succeeded.)
• Ran unit tests for xtask with cargo test -p xtask, which executed the new snapshot/validation tests; all tests passed (98 passed, 0 failed). (Succeeded.)

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: fed71f37-e073-4db7-96f9-79814a4b9cb2

📥 Commits

Reviewing files that changed from the base of the PR and between 449adfa and d4db0d1.

📒 Files selected for processing (7)

• README.md
• docs/how-to/publishing.md
• docs/how-to/release.md
• docs/metadata/workspace-docs.json
• docs/reference/support-matrix.md
• xtask/src/docs_sync.rs
• xtask/src/main.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/define-crate-support-matrix-and-metadata-2xvq84

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