Give plugin errors W codes and module resolution M036 (#1968)

[shunichironomura]

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Summary

Fixes #1968: several stable diagnostic codes were each assigned to two different errors.

• Plugin errors move from P0nn to W0nn (numbers kept, whole family including P010). P stays with parse errors, which are older and more numerous.
• ModuleError::ModuleResolution moves from M033 to M036. LoadError::FileRootSelfImport keeps M033, since it was assigned first (Reject file-root self-imports #1532); the collision came in with Give compile-time diagnostics their own codes and typed payloads (follow-up S14–S15) #1933.
• New workspace guard crates/graphcal-project/tests/diagnostic_codes.rs: discovers every code assignment in non-test sources across crates/*/src (code() arms, miette code(...) attributes, CODE constants) and asserts each code has exactly one site and each file uses one prefix. New error enums are covered without registration. Verified it fails on the reintroduced M033 collision. The compiler-only uniqueness loop is removed; its pinned-code test also pins M036, W001, W010.
• Regenerated CLI output goldens and insta snapshots (code strings only), updated one LSP assertion, plugin-code comments, and the reading guide.
• Docs (en/ja): extern-functions.md cites the W codes. authoring-plugins.md said P005/P016; the bare-binding rule actually reports InvalidExternSignature, so it now says W001/P016.

Out of scope: LSP-emitted literal codes (A001, and the deliberate E001 reuse) are not enum definitions and are not scanned.

AI assistance

• AI used: yes — Claude Code investigated the issue, implemented the change, and ran just lint / just test.
• Human review of AI-assisted code: pending

Breaking changes

• No breaking changes
• Breaking changes; the breaking change label is applied