drop OBJ_VERSION_NO_HEADER_AAD (pack object format v1) support, fixes #9973

[ThomasWaldmann]

Fixes #9973.

RepoObj.parse() / parse_meta() accepted two pack object format versions: OBJ_VERSION_NO_HEADER_AAD (0x01, only ever written by 2.0.0b22) and OBJ_VERSION_HEADER_AAD (0x02, what format() writes since b23). We don't owe compatibility across betas, so the 0x01 fallback is gone before rc1:

• repoobj.py: OBJ_VERSION_NO_HEADER_AAD removed, SUPPORTED_OBJ_VERSIONS is now (OBJ_VERSION_HEADER_AAD,), the version-dependent AAD selection in parse() / parse_meta() is gone (header AAD + slot tag always). Comments and the object_validator docstring updated accordingly.
• repoobj_test.py: test_version1_object_without_header_aad_still_readable removed.
• docs/internals/packs.rst: the 0x01 paragraph and the "0x01 still readable" table entry removed.

A 0x01 object now fails with unsupported object version: 1 (the existing version check), same as any other unknown version.

Full test suite passes locally (one unrelated TestTimeoutTimer timing flake under -n auto, passes alone).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.04%. Comparing base (11d7d10) to head (0fc165b).
⚠️ Report is 7 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10362      +/-   ##
==========================================
+ Coverage   87.93%   88.04%   +0.11%     
==========================================
  Files         103      103              
  Lines       18893    18912      +19     
  Branches     2917     2919       +2     
==========================================
+ Hits        16613    16651      +38     
+ Misses       1583     1569      -14     
+ Partials      697      692       -5     

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