create/import-tar --json: report the deduplicated size of the new archive, #10335

[ThomasWaldmann]

Fixes #10335.

The claim in the issue is correct: Statistics.usize is accumulated while creating an archive and printed as Deduplicated size: in the text --stats block, but Statistics.as_dict() did not include it, and --json replaces the text block, so a JSON consumer could not get the deduplicated size at all. borg info can not recover it later (calc_stats() does not compute it, that is expensive).

Changes

• Statistics.as_dict() reports deduplicated_size (the borg 1.x key; in borg 2 it is the deduplicated original size, see the 2.0.0a2 changelog). This covers borg create --json, borg import-tar --json and the archive_progress JSON lines (which already show the U size in text mode).
• Archive.calc_stats() (used by borg info) marks usize as unknown (None) instead of 0, and as_dict() leaves the key out then. So info --json stays honest: absent means "not computed", not "zero" (option 2 from the issue).
• Docs: docs/internals/frontends.rst (keys, examples, the borg info note), docs/changes.rst, and the create / info help texts (docs/usage/*.rst.inc regenerated, only these two files changed). The create help still described a borg 1.x "This Archive" / "All archives" table that borg 2 does not print; fixed while there.
• Tests: Statistics.as_dict() unit test, create --json (fresh repo vs. repeated backup), import-tar --json, and info --json not having the key.

Sample output (borg from this branch)

$ borg create --json arch input          # fresh repository
"stats": {"deduplicated_size": 150504, "original_size": 150504, "nfiles": 2, ...}
$ borg create --json arch input          # unchanged input again
"stats": {"deduplicated_size": 469, "original_size": 150504, "nfiles": 2, ...}
$ borg create --stats arch input         # the text block agrees
Deduplicated size: 470 B
$ borg info --json --last 1              # no deduplicated_size key
"stats": {"chunking_time": 0.0, "files_stats": {}, "hashing_time": 0.0, "nfiles": 2, "original_size": 150035}
$ borg create --progress --log-json ...
{"original_size": 0, "deduplicated_size": 0, "nfiles": 0, ..., "type": "archive_progress", "finished": false}

The --dry-run --json output is unchanged (nfiles and original_size only, nothing is deduplicated there).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.75%. Comparing base (95856d4) to head (607ab62).
⚠️ Report is 9 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10336      +/-   ##
==========================================
+ Coverage   87.74%   87.75%   +0.01%     
==========================================
  Files         103      103              
  Lines       18817    18821       +4     
  Branches     2905     2906       +1     
==========================================
+ Hits        16511    16517       +6     
+ Misses       1602     1600       -2     
  Partials      704      704              

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[ioanalytica]

Great. Thanks