The promotion worker mounts the growing SMB share, but a CIFS soft-mount can
stay mounted while DEAD (server dropped the connection) — every access then
returns ENOENT, so promotion fails Growing file not found and the asset is
stranded in processing (recurring stuck-migration bug). Fixes:
- ensureGrowingShareMounted now PROBES the mount with a readdir; if dead, lazy-
unmounts and remounts fresh (was: returned early if anything was mounted).
- file lookup retries for ~20s (CIFS attribute-cache lag on a freshly written
master), remounting between attempts.
- on any promotion failure, the asset is reset (pending_migration if the file is
present, else error) instead of being left in processing forever.
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