Guard multiprocessing logging by the active start method

[Shubham-Padkonde]

Fixes #106.

multiprocessing-logging requires the fork start method. The existing Windows-only guard still attempts to install its handler with macOS's default spawn, or explicitly selected spawn/forkserver on Linux, causing an assertion instead of starting logging.

Check the active start method before installing the handler. Unsupported methods emit a warning and retain ordinary logging; fork remains supported. The check does not run when multiprocessing logging is disabled, and does not change the application's start method. Both public logging docstrings describe the fallback.

Tests cover both unsupported methods, successful handler installation with fork, and the opt-out path. The two unsupported-method regressions fail before the fix and pass afterward. I also reproduced the dependency assertion with an actual spawn context before the fix.

Validation on Linux / Python 3.13.14 with the git and multiprocessing extras:

• 24 functional tests passed (pytest -q -k 'not benchmark').
• Separate actual spawn, forkserver, and fork processes wrote a log, produced the expected warning behavior, and retained their selected start method.
• Full pre-commit run --all-files passed, including Ruff, mypy, check-manifest and codespell.
• The full suite has one existing timing-benchmark failure here. Its 0.13-second threshold is exceeded on both base and changed code, including a native Linux filesystem copy (base: 0.140s; changed: 0.148s). The benchmark and threshold are unchanged.

Native macOS/Windows tests were not run. Submitting as a draft for discussion in line with the NIU contribution guide. Prepared with Codex assistance.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.35%. Comparing base (95ed0e9) to head (5128d7b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #107      +/-   ##
==========================================
+ Coverage   78.12%   80.35%   +2.23%     
==========================================
  Files           3        3              
  Lines         224      224              
==========================================
+ Hits          175      180       +5     
+ Misses         49       44       -5     

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