fix(ci): resolve pytest hang after tests pass (loguru __del__ deadlock)

[incrementaliser]

Problem

CI reported success (191 passed, 2 skipped in ~4.5s) then stalled until the job timeout (~45 minutes). The hang occurs after pytest finishes, during interpreter shutdown.

Root cause

InteractiveContextParser.__del__ called loguru.remove() for each parser's sinks. When several parsers with active logging are garbage-collected together (common after the loguru integration and increased parser usage from parallel language-derivation tests), loguru's internal lock is re-entered from nested __del__ calls and the main thread deadlocks.

This reproduces locally with:

uv run pytest tests/test_icp_logging.py::test_log_level_error_captures_unknown_word \
  tests/test_icp_logging.py::test_log_level_warning_suppresses_info

Parallelisation/Rich/ProcessPoolExecutor mitigations on main were reasonable hardening but did not fix this primary failure mode.

Fix

• Remove __del__ sink cleanup; add explicit close() / _remove_icp_log_sinks().
• Restore spawn multiprocessing context for ProcessPoolExecutor in language derivation (safe for real parallel runs outside pytest).
• Add a small regression test for the two-parser terminal-log scenario.

Verification

• Full suite: uv run pytest -q → 191 passed, 2 skipped and exit code 0 (previously hung with exit 124 under timeout).