Defense-in-depth follow-up to #760. Even with the entrypoint chown fix, if the
album-bundle staging dir ever can't be created/written (permissions, read-only
mount, disk full), the dispatch caught the plugin exception and marked the whole
batch failed — even though the album had already downloaded (the #715 symptom:
'release finishes downloading but the batch fails').
Now an OSError from the plugin is flagged fallback-eligible, so the dispatch
returns to the per-track flow instead of hard-failing. OSError covers the
staging/filesystem failure that motivated this (#760's PermissionError) and, by
Python's IOError==OSError aliasing, any propagated transient I/O error —
falling back is never worse than hard-failing, and per-track is the universal
graceful path. Programming errors (TypeError, KeyError, RuntimeError, …) are
NOT OSError and stay terminal, so genuine bugs still fail loudly — the existing
'plugin exception => failure' contract and its test are preserved.
Test: new test_dispatch_staging_oserror_falls_back_to_per_track (PermissionError
on the staging dir -> result False, phase 'analysis', not failed). Existing
RuntimeError-is-terminal test still passes. 131 album-bundle/plugin tests green.
c8c3789cb9