* feat(cache): add completion marker helpers to spreadFS
* feat(cache): write completion marker after successful cache write
* fix(cache): adopt only complete files on reload, grandfather existing caches
* test(cache): regression tests for partial-transcode crash leftover (#5636)
* test(cache): guard concurrent in-progress streaming with completion marker
* test(cache): make concurrent-streaming guard actually attach a second reader mid-write
The previous test obtained s2 only after pw.Close(), so no reader ever
attached to the in-progress entry. Now pw.Write("hello ") is called
synchronously before the second Get — io.Pipe's blocking write gives a
deterministic happens-before — then both s1 and s2 are drained in parallel
goroutines while the producer writes the rest and closes the pipe.
* style(cache): clarify best-effort intent of cleanup os.Remove calls
* refactor(cache): lift one-time grandfather pass out of Reload's steady-state loop
* refactor(cache): have MarkComplete take the key, owning path mapping in spreadFS
* test(cache): assert no completion marker is written when the write fails
* refactor(cache): rename migration sentinel to generic .nd-migrated
* refactor(cache): rename grandfather migration to migrateExistingFiles
* refactor(cache): single-pass Reload with safer marker-error handling
Address PR review feedback:
- Merge the one-time migration into Reload's single directory walk,
avoiding a second full walk on first boot.
- Only delete a data file when its marker is definitively absent
(os.IsNotExist); skip on other stat errors to avoid destroying valid
entries under transient I/O failures.
- Write the migration sentinel only after a clean walk, so a partial
walk can't strand valid-but-unmarked files for later deletion.
- Return early from walkDataFiles on a WalkDir error.
- Assert fs.Create error in the marker-removal test.
