fix: distinguish completed and unresumable backup files

[ivanauth]

Summary

OcfFileEncoder.Cursor() inferred "completed" from a missing .lock, but .lock is only written on the first Append() — so a backup that crashed before the first relationship was misreported as completed and refused to resume.

Add a .done sentinel written on successful Close(). Cursor() now returns the cursor when .lock exists, ErrBackupAlreadyCompleted when only .done does, and ErrBackupUnresumable when neither. Close() writes .done before removing .lock. NewFileEncoder clears stale sidecars on fresh open.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 51.85185% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.97%. Comparing base (b0cdc17) to head (0dc6d91).

Files with missing lines	Patch %	Lines
pkg/backupformat/encoder.go	51.85%	8 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #694      +/-   ##
==========================================
+ Coverage   43.77%   43.97%   +0.20%     
==========================================
  Files          38       38              
  Lines        6193     6212      +19     
==========================================
+ Hits         2711     2732      +21     
+ Misses       3201     3195       -6     
- Partials      281      285       +4     

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