diff --stats follow-up: size_added/size_removed naming, JSON docs, test timing

[ThomasWaldmann]

Follow-up to #10327 (#796), addressing three review findings:

• Naming: "Added chunk volume" / "Removed chunk volume" and the JSON keys
  added_chunk_volume / removed_chunk_volume introduced new vocabulary for a quantity the rest of
  borg diff already names: the totals are the sums over all reported items of the per-path
  added / removed byte counts, i.e. of what --sort-by size_added / size_removed sorts by.
  The lines are now "Added size" / "Removed size" and the JSON keys size_added / size_removed.
  diff: add --stats, showing a summary of the differences #10327 is unreleased, so nothing depends on the old names.
• Frontend docs: docs/internals/frontends.rst documents the borg diff --json-lines output for
  frontend authors, but did not mention the --stats summary line yet. It now describes the
  {"stats": {...}} line and its six keys, with an example matching the existing one.
• Test timing: test_stats_counts_metadata_only_changes expects the input directory to be reported
  as changed (mtime/ctime), which requires the two archives to be created on different timestamp ticks.
  The setup helper did not call granularity_sleep() like the other tests in the file do; on HFS+
  (whole-second timestamps) the test failed 8 out of 8 runs with "Changed items: 2". It now sleeps
  between the two archives.

Verified locally: ruff and black, the diff test module (26 passed, binary variants skipped), and a
warning-free Sphinx build of the docs.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.79%. Comparing base (366db25) to head (b4ae613).
⚠️ Report is 27 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10339      +/-   ##
==========================================
- Coverage   87.80%   87.79%   -0.01%     
==========================================
  Files         103      103              
  Lines       18828    18868      +40     
  Branches     2906     2914       +8     
==========================================
+ Hits        16531    16566      +35     
- Misses       1598     1599       +1     
- Partials      699      703       +4     

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