feat: propagate historical workers and clarify NDWS caveats

[peetmate]

Summary

• propagate bounded historical --workers controls across historical fetch, statistics, season, compare, climatology, hazard, and station-vs-grid public surfaces
• carry worker support through package-native GEE/Xee batch paths while preserving live coverage clipping metadata
• improve NDWS/WRSI user-facing interpretation by surfacing methodology notes and threshold caveats in reports and hazard outputs

Why

• historical CLI/module users need one consistent way to tune bounded Xee worker counts
• merged main coverage/date metadata in batch fetch needed to coexist with worker propagation
• NDWS day-count outputs in perhumid or fallback fixed-window cases needed explicit caveats so hazard labels are not over-read

Validation

• .venv/bin/python -m pytest -q tests/test_historical_workers_surface.py tests/test_fetch_pipeline.py tests/test_gee_xee_batch.py tests/test_xee_common.py
• .venv/bin/python -m pytest -q tests/test_compare_periods_baseline.py tests/test_statistics_source_policy.py
• .venv/bin/python -m pytest -q tests/test_hazard_thresholds.py tests/test_compare_periods_baseline.py tests/test_statistics_source_policy.py

Issues

• advances #85
• advances #4