find: -printf: fix %A+/%C+/%T+ fractional seconds to match GNU

[leeewee]

-printf %A+/%C+/%T+ diverged from GNU on low-precision timestamps. The %+ format expanded to …%S%.f0, using chrono's %.f, which trims trailing zeros and omits the fraction entirely when it is zero. GNU always writes a fixed-width fraction (snprintf(".%09ld0", tv_nsec) in find/print.c).

Before:

$ touch -d @1700000000 a          # whole second, ns = 0
$ find a -printf '%A+\n'
2023-11-14+22:13:200              # missing ".NNNNNNNNN"
$ /usr/bin/find a -printf '%A+\n'
2023-11-14+22:13:20.0000000000

After — matches GNU byte-for-byte across ns = 0 / microsecond / full precision:

$ find a -printf '%A+\n'
2023-11-14+22:13:20.0000000000

The fix uses .%f (literal dot + fixed 9-digit %f), like the %S and ctime paths already do. As a side effect it makes the existing find_printf test's %A+ assertion deterministic (always .NNNNNNNNN0), which removes the intermittent Windows CI failure seen on that test.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.88%. Comparing base (6ceb2a7) to head (98bfad2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #757   +/-   ##
=======================================
  Coverage   91.88%   91.88%           
=======================================
  Files          35       35           
  Lines        7159     7159           
  Branches      377      377           
=======================================
  Hits         6578     6578           
  Misses        437      437           
  Partials      144      144           

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[github-actions]

Commit 98bfad2 has test result changes:

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 315 / PASSED: 267 / FAILED: 42 / SKIPPED: 6
  Reference: TOTAL: 313 / PASSED: 267 / FAILED: 40 / SKIPPED: 6

Changes from main branch:
  TOTAL: +2
  PASSED: +0
  FAILED: +2

New test failures (2):
  - gnu/files0_from_ok
  - gnu/okdir_path_relative

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

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_{Comparing leeewee:find-fix-printf-plus-fractional (98bfad2) with main (6ceb2a7)}