Add docstrings to 8 nested test helpers to restore 100% docs coverage

[tschm]

Subcategory: Docstring / API-doc coverage
Score: 8 → 10

make docs-coverage runs interrogate --fail-under 100, so any missing docstring fails the gate. Coverage is currently 97.4%. All 8 gaps are nested helper classes/functions defined inside test bodies — src/ is fully documented.

What to change

• tests/pycharting/api/test_interface.py:163 — class _Event
• tests/pycharting/api/test_interface.py:167 — _Event.is_set
• tests/pycharting/api/test_interface.py:171 — _Event.wait
• tests/pycharting/api/test_interface.py:191 — class _InterruptingEvent
• tests/pycharting/api/test_interface.py:192 — _InterruptingEvent.is_set
• tests/pycharting/api/test_interface.py:195 — _InterruptingEvent.wait
• tests/pycharting/core/test_lifecycle.py:284 — def _healthy
• tests/pycharting/core/test_lifecycle.py:305 — def _healthy

done when

make docs-coverage reports 100.0% and exits 0.

Evidence

RESULT: FAILED (minimum: 100.0%, actual: 97.4%)
| test_plot_block_returns_when_shutdown_event_set._Event (L163)  |  MISSED |

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Surfaced by the Rhiza quality gates during the template bump to jebel-quant/rhiza@v1.2.2, which enabled the stricter ruff rule families (ANN, A, BLE, ARG) and TYPECHECKER=both. Gate status at time of filing: make fmt FAIL, make typecheck FAIL, make docs-coverage FAIL, test-layout FAIL; make deptry, make security, make test PASS (154 tests, 100% coverage).