MAINT: Use sysmon for coverage

[Roman Lutz (romanlutz)]

Description

Reduce coverage measurement overhead by setting COVERAGE_CORE: sysmon only on the existing CI coverage test step. The pinned coverage.py version supports this project's line coverage on Python 3.12; no branch coverage, dynamic contexts, or incompatible plugins are configured.

The measured source, four-worker loadfile scheduling, 78% overall and 90% diff coverage gates, reports, dependencies, and runner/Python versions are unchanged.

Tests and Documentation

• Both cores passed the same 40 existing tests on Windows/Python 3.12 with four xdist workers. Actual SysMonitor selection was confirmed in all workers.
• Executed, missing, and excluded line sets matched across all 528 files (4,365/36,848 covered lines); XML reports matched except for timestamps. The subset correctly failed the unchanged 78% gate, and boundary probes verified passing and failing behavior for both coverage gates.
• Workflow YAML and applicable pre-commit hooks passed. Full Linux CI performance has not been measured. No documentation changes or JupyText run are needed for this workflow-only change.