write chunkindex fragment(s) before the archive pointer

[ThomasWaldmann]

Fixes #10239.

The archive pointer write (archives/<archive_id>) is the commit point of a backup. Previously, the chunkindex fragment covering the session's new chunks was only written at cache close (and every 600s during the backup), i.e. after the pointer: a crash in between left a committed archive whose chunks were partly not covered by index/*, so reads (which are routed through the index) raised ObjectNotFound until borg check rebuilt the index.

Now Archive.save() and Archive.set_meta() (rename/tag/copy) persist the index fragment(s) covering everything the session stored so far before writing the pointer, so a committed archive always has complete index coverage. A crash before the pointer write merely leaves an index fragment referencing uncommitted objects, which is harmless (compact/rebuild prunes it). This also covers import-tar, recreate and transfer, which commit archives through Archive.save(); the check --repair paths already rebuild the whole index in finish().

Notes:

• The forced write resets the periodic (600s) index write timer, so no duplicate write follows right after committing.
• The close-time index write is now usually a no-op (no new chunks left), but is kept for sessions that store chunks without committing an archive.
• Multi-archive sessions (e.g. borg transfer) write one fragment per committed archive, covering the chunks stored since the previous write; the bounded repack at cache close consolidates small fragments as before.
• docs: removed the packs.rst TODO about this and updated the write-order steps to match the implementation (the figure already showed the intended order).
• test: regression test simulates dying right after the commit point (close-time index write suppressed) and asserts the archive is still fully readable via index/* only. It fails on master: there, the archive is not even findable, as its metadata chunk cannot be read through the index.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.47%. Comparing base (6299333) to head (c82a280).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10248   +/-   ##
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  Coverage   87.47%   87.47%           
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  Files         103      103           
  Lines       18534    18538    +4     
  Branches     2843     2843           
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+ Hits        16213    16217    +4     
  Misses       1623     1623           
  Partials      698      698           

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[ThomasWaldmann]

@mr-raj12 ^