1.0.0-rc.6

Release candidate patch for ArtifactFS 1.0.

This supersedes 1.0.0-rc.5 and hardens concurrent filesystem updates, repository lifecycle management, Git object streaming, and blob cache safety.

Changes since 1.0.0-rc.5:

- Keep open FUSE file handles valid across unlink and rename, serialize namespace mutations, and safely promote committed files into the writable overlay.
- Make overlay reconciliation, snapshot publication, generation cleanup, SQLite access, deletes, and fsync behavior coherent and durable under concurrency.
- Track, cancel, and join mount, refresh, hydration, verification, preparation, retry, and watcher workers cleanly during shutdown.
- Bound persistent `git cat-file --batch` concurrency, reject malformed or desynchronized Git responses, and discard failed batch processes.
- Stream blob bytes through identity-verified descriptors and atomically replace cache entries without exposing partial or substituted content.
- Detect ref changes by content, serialize HEAD updates, preserve staged work, and retry only recognized transient Git transport failures.

Verification:

- PR #30 Build & Test, CodeQL, and Semgrep checks passed.
- Local release validation: `go build ./cmd/artifact-fs`, `go vet ./...`, `go test ./...`, and `go test -race ./internal/...`.