create: support --stats with --dry-run

[ThomasWaldmann]

Fixes #1648, fixes #4131.

--stats (and --json) now work together with --dry-run for borg create.

A dry run already walks all files and stat()s them, so it can cheaply count the files and sum up their sizes from file system metadata, without reading any file contents — the dry run stays as fast as before.

Text output:

Number of files: 3
Original size: 1.33 MB

--json returns "dry_run": true and a reduced stats object (nfiles, original_size) instead of the archive object (documented in frontends.rst).

Notes on the numbers:

• The deduplicated size is unknown in a dry run, as data is not read, chunked, and deduplicated.
• Sizes of data read from stdin, from a command's output (--content-from-command), and from special files (--read-special) are unknown and counted as zero.
• Hardlinks count once per link, as in a real create.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 53.57143% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.98%. Comparing base (e15404a) to head (448f1c6).
⚠️ Report is 11 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/archiver/create_cmd.py	50.00%	12 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10105      +/-   ##
==========================================
- Coverage   87.01%   86.98%   -0.03%     
==========================================
  Files          99      100       +1     
  Lines       17615    17684      +69     
  Branches     2664     2674      +10     
==========================================
+ Hits        15327    15382      +55     
- Misses       1593     1606      +13     
- Partials      695      696       +1     

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