fix(cpp): degrade gracefully when progress Manager cannot spawn (#339)

[breimanntools]

Closes #339.

Summary

CPP.run(..., n_jobs>1) crashed in non-interactive contexts (python -c, heredocs, some subprocess
shells) where multiprocessing.Manager() raised EOFError/OSError while setting up the
cross-process progress bar. The only workaround was n_jobs=1.

Details

• _get_mp_shared() now creates the Manager best-effort: on failure it leaves all module globals
  untouched (no half-initialized state, no bumped refcount, half-started Manager shut down) and
  returns None, so _resolve_shared() degrades to the thread-safe DEFAULT_SHARED_* progress path.
• Because the assign / pre-filter dispatch uses joblib's process backend, a new _worker_shared()
  helper hands Manager proxies to workers unchanged (byte-identical happy path) but passes None for
  the thread-safe fallbacks (a threading.Lock cannot be pickled to a spawned process), so each
  worker self-resolves its own process-local progress and the run completes.
• Progress is cosmetic-only; output is unchanged when the Manager is available.

Ripple

• tests/unit/cpp_tests/test_progress_backend.py — Manager failure/degradation arm, clean-globals
  invariant, resolve fallback, and an end-to-end CPP.run(n_jobs=2) under a forced Manager failure.
• release-notes Unreleased Fixed entry.

Part of epic #336.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.83%. Comparing base (7dcc8d8) to head (3e601f9).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
...ure_engineering/_backend/cpp/_filters/_progress.py	90.90%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #348   +/-   ##
=======================================
  Coverage   94.83%   94.83%           
=======================================
  Files         196      196           
  Lines       18767    18787   +20     
  Branches     3175     3177    +2     
=======================================
+ Hits        17797    17817   +20     
- Misses        633      635    +2     
+ Partials      337      335    -2     

Files with missing lines	Coverage Δ
...ature_engineering/_backend/cpp/_filters/_assign.py	98.85% <100.00%> (+<0.01%)	⬆️
..._engineering/_backend/cpp/_filters/_stat_filter.py	92.72% <100.00%> (+0.04%)	⬆️
...ure_engineering/_backend/cpp/_filters/_progress.py	85.50% <90.90%> (+5.11%)	⬆️

Components	Coverage Δ
cpp_core	95.08% <91.66%> (+0.13%)	⬆️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.