Simple immutable behavior: inject error → fsync fail → immutable sticky Covers: multiple write+fsync success cycles, then failure, repeated fsync fail

Consolidated mixed read-write test (without immutable): 1) Multiple write + fsync + read-back cycles with data verification 2) Release + reopen + read

Consolidated mixed read-write test with immutable: 1) Multiple write + fsync success cycles building up data 2) Append + inject error + fsync fail ->

Verify block cache retention for small file when one handle closes while another handle still active. Ensures cache is not completely evicted in multi