ArtifactFS 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/....