Implement staging storage failure and rollback semantics

[niteshpurohit]

Objective

Apply Laghu's existing filesystem failure contract specifically to the staging store.

Acceptance Criteria

Handle safely:

• ENOSPC;
• inode exhaustion;
• permission denial;
• read-only mount;
• failed fsync/rename;
• missing/unavailable staging mount;
• process restart with incomplete state.

No fallback directory or unbounded memory buffering is permitted.

Technical Considerations

• Any affected /files transaction fails atomically.