feat(catalog): implement stable content index

[zoeyrose]

Summary

• define stable, namespaced catalog identities and typed edges for all nine authored-content domains
• add deterministic cross-file indexes for definitions, aliases, inheritance, references (including preview media), reverse dependencies, previews, and bounded search
• add revision-aware immutable updates that patch touched index contributions, report exact bounded transitive invalidation, and match clean builds
• expand shared diagnostics with source locations, related references, semantic paths, fix hints, bounded suppression, and stable catalog codes
• add a revision-bound, baseline-gated whole-corpus catalog/reference audit beside the lossless source audit
• document the shared CLI/CI/editor catalog and diagnostic contracts

Validation

• tools/validate.sh
• actionlint .github/workflows/*.yml
• cargo test --locked --workspace --all-targets --all-features
• cargo clippy --locked --workspace --all-targets --all-features -- -D warnings
• pinned catalog corpus: 5,678 documents; 33,372 definitions across all nine domains, including 15,453 resources and 9,413 faces; 254 owned diagnostics with a pinned identity digest; no truncation; exact baseline match
• git diff --check

Verification and cleanup

• profile: issue-4-stable-catalog; worktree: /workspaces/atrinik/workspace/worktrees/content-toolkit/issue-4-stable-catalog
• this is a headless toolkit change; replacement components expose no wrapper build/runtime adapter, so no topology, service, mutable state, or scenario is needed
• the validation commands above are safe to repeat and create only repository-local build outputs
• keep the open-PR worktree; any post-merge cleanup is a separate request starting with ./atrinik cleanup --dry-run --json

Boundaries

• provenance and license fields remain opaque evidence references; this change makes no legal or ownership determination
• the catalog consumes parsed documents and does not retain resource payloads or implement the later ABIN/AMAP artifact milestones
• follow-up fix(validation): reconcile catalog reference baseline content#215 owns reconciling the remaining 245 required and 9 optional missing authored references
• no authored content changes are required for this library/API issue

Closes #4

[github-actions]

🎉 This PR is included in version 2.1.0 🎉

The release is available on:

• v2.1.0
• GitHub release

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