Implement Phase C: Production Hardening

Implements phase-c-hardening.md to fix 6 RED resilience tests:

- D1/D2: Health check timeout (1.5s) + parallel execution via join_all
- C6: Recursive CAS calculate_size() to scan shard subdirectories
- C7: FUSE read timeout (30s) returns EIO instead of hanging
- 6.4: Auto-re-fetch corrupt/missing chunks from origin
- 6.6: Passthrough mode - continue even when CAS write fails
- C9: PID file with flock prevents concurrent mounts
- 5.3: fd exhaustion handling test

All 27 resilience tests now pass. Full test suite green.

Files changed:
- musicfs-origins/src/health.rs: timeout + join_all
- musicfs-origins/Cargo.toml: add futures dependency
- musicfs-cas/src/store.rs: recursive calculate_size
- musicfs-cas/src/reader.rs: auto-re-fetch on IntegrityError/NotFound
- musicfs-cas/src/fetcher.rs: passthrough fallback
- musicfs-fuse/src/filesystem.rs: 30s read timeout
- musicfs-cli/src/main.rs: PID file with flock
- musicfs-test-utils/tests/resilience.rs: updated tests

musicfs/Cargo.lock

@@ -1929,6 +1929,7 @@ dependencies = [
  "anyhow",
  "clap",
  "dirs",
+ "libc",
  "musicfs-cache",
  "musicfs-cas",
  "musicfs-core",
@@ -2018,6 +2019,7 @@ version = "0.1.0"
 dependencies = [
  "async-trait",
  "dashmap",
+ "futures",
  "libc",
  "musicfs-core",
  "parking_lot 0.12.5",