repository: read-only check by hashing pack/index objects

[mr-raj12]

Makes the repository part of borg check work with packs (N>1).

packs/ and index/ objects are named by the sha256 of their content, so check just verifies store.hash(name) == name for each one. It doesn't parse pack headers at all, so a pack with many objects is checked exactly like a pack with one. On REST the hash is done server-side, so nothing is downloaded there; other backends still download the object to hash it.

The chunk index is not rebuilt anymore. A read-only check shouldn't write, and rebuilding from pack headers is unsafe: one corrupt header would write wrong offsets for every later object in that pack. Repository repair isn't done yet, so for now it runs the same read-only check and reports problems without dropping any pack (dropping it would lose the pack's other, good objects too).

Adds a progress indicator over packs/ and index/.

The archives check rebuilds its index from pack headers read-only (disable_caches) so it matches the actual packs and can still spot missing chunks. Note this is on purpose and not what #8476 suggests for the repository check: the repository check above does reuse the existing index objects, but the archives check needs an index that reflects the real packs, otherwise a stale cached entry would hide a chunk whose pack is gone. The scan reads headers only and writes nothing back.

Tests cover corruption in a later object, index corruption, an intact multi-object pack, and the progress wiring. check_cmd_test.py: 24 passed, 27 skipped.

refs #8476 #8572

[codecov]

Codecov Report

❌ Patch coverage is 93.02326% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.48%. Comparing base (0ece418) to head (dc5aa66).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/repository.py	92.85%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9794      +/-   ##
==========================================
- Coverage   84.70%   84.48%   -0.23%     
==========================================
  Files          92       92              
  Lines       15172    15142      -30     
  Branches     2279     2271       -8     
==========================================
- Hits        12852    12792      -60     
- Misses       1617     1648      +31     
+ Partials      703      702       -1     

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[ThomasWaldmann]

i only looked at archive and repository code, not tests.