Add hardness, water-type, data-density, and water-level-change products

Four new OGC GeoJSON products, all reusing existing cohort source fetches
(zero new API pulls):

- nm_hardness (ogc_hardness): total hardness as CaCO3 from latest calcium +
  magnesium, with soft/moderate/hard/very-hard class.
- nm_water_type (ogc_water_type): hydrochemical (Piper) water type from the
  major-ion suite — meq% of cations (Ca/Mg/Na+K) and anions (HCO3+CO3/Cl/SO4),
  dominant ion >50% else mixed, plus charge-balance QC.
- nm_waterlevel_data_density (ogc_data_density): per-well measurement coverage
  (raw/daily counts, span, mean interval, observations per year).
- nm_waterlevel_change (ogc_waterlevel_change): depth-to-water change over a
  trailing window (default 5y); positive = water table declined; start must be
  within half the window of the target else status=insufficient.

Hardness and water-type pivot per-well summaries (analytes cohort); data-density
and water-level-change consume sites/timeseries (waterlevels cohort). All four
add no new shared source assets — they ride fetches the existing products
already perform.

Adds the dumpers + shared pivot/feature helpers in ogc_features.py, the combine
dispatch and multi-analyte param handling in products.py, the output types in
definitions.py, the product entries in products.yaml, and dumper tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: jirhiker