feat(http-server): add durable repository projections

[forhappy]

Summary

• add durable per-repository path-state and attribution projections backed by Cell/SQLite state
• expose projection-aware tree attribution reads with readiness, lag, repair scheduling, and metrics
• make repository Cell routing lifecycle-safe under concurrent reads, scheduler work, migration, and drain
• integrate path-state metadata into Git snapshots, commit history, object reads, GC/repack, and the repository browser
• document the serverless Git projection architecture and add architecture diagrams
• preserve bounded runtime admission, including 16 MiB per-Cell mailbox capacity and node-wide retained-byte limits

Validation

• cargo test -p crab-http-server --lib --locked -- --test-threads=1 — 199 passed, 4 ignored
• cargo check --workspace --locked
• cargo clippy -p crab-http-server --lib --locked -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• packages/repository: 46 Vitest tests, TypeScript typecheck, Prettier check
• real RustFS-backed HTTP/Git smoke: projection recovered to ready, tree-attribution returned 200, Git clone/fetch and git fsck passed; 8/16-concurrent attribution requests completed without errors, overload returned bounded 429s instead of 5xx