v0.5.1 — idempotent annotation upsert

Patch: idempotent upsert for cache-class / system-derived annotations.

• annotate --replace — upsert by (type, key) for a symbol (delete-then-write). Append-only stays the default for advisory/note/comment/question; --replace is opt-in for re-projectable cache tags so a re-project doesn't duplicate rows.
• clusters --annotate is now idempotent — re-running replaces each symbol's community annotation instead of duplicating it.

Unblocks consumers that project a re-buildable cache as annotations. Gate green: build 0 warnings, 44 test suites, clippy -D warnings, fmt.

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