All three themes on the faceted engine: Need/Hazard, Event activation picker, geocode Location

[BhattaraiSijan]

Parent: #164 (LayerFilter plugin). Depends on: filter foundation (merged).
Activation-selection side effects (default-on / fly-to / time) are out of
scope — pending decisions tracked in SCHEMA.md §8; this UI ships without them.

Problem

With the machinery in place, the visible product remains: all three theme
panels running on the engine. Need and Hazard still run the old flat
matcher with hand-enumerated dropdown options (a second copy of the
taxonomy that drifts from the data). The Event theme — facets over the
activation catalogue, an adapting activation picker, results = the picked
activation's layers — doesn't exist at all. And its Location filter is
defined as a geocode search over activation geometries (no location field
exists anywhere), following the same external-geocoder pattern already
used by the map-control search.

Outcome

Need & Hazard on the engine:

• Dropdowns populated from distinct values found on the mission's layers
  (display-ready values, arrays), each option with its matching-layer
  count; zero options in configuration; the old per-layer theme/location/
  year fields are gone and ignored.
• Selections narrow the layer panel via the listed flag; persist per theme
  across switches; result counts match the design's "N Results".

Event theme:

• Hazard-type and Year facets plus the activation picker, all mutually
  adapting per the cascade; "N activations" reflects current facets; each
  option carries a year badge; the selected activation shows its date
  range and an ACTIVE pill when ongoing.
• Picking an activation narrows the layer panel to its layers; narrowing
  facets past the selection auto-clears it and un-narrows.
• Side effects (auto-toggle default-on layers, fly-to, time) are out of
  scope, pending the SCHEMA.md §8 decisions — pairing attributes flow through
  untouched.

Location (geocode) facet:

• Typeahead search against the configured geocoder; chosen place shown as
  a clearable chip; activations pass by geometric intersection (polygon
  results intersect properly; point-geometry activations match containing
  regions). No geocoder configured → the control hides; theme still works.
• Outward-only cascade accepted and documented: other facets don't narrow
  free text, but counts always reflect the full selection.

Acceptance

• Demo config: sector/hazard dropdowns match the distinct tags with
  correct counts and update the panel live; a two-sector layer appears
  under both without duplication; adding a tag to a layer config and
  reloading adds the option with no plugin config change.
• Year=2024 collapses hazard/activation options to 2024-consistent
  values while Year itself keeps all years listed.
• Picking an activation shows its layers and narrows the panel; an
  ongoing activation shows ACTIVE; flipping Year to a non-matching
  value resets the picker to placeholder.
• Geocode: picking a state filters the demo catalogue correctly
  (statewide polygon matches, out-of-state point doesn't); clearing
  the chip restores; endpoint from configuration only
  (grep-verifiable); unreachable geocoder degrades with a message.
• Theme switching Need → Hazard → Need restores the sector selection.
• Token-based styling (USWDS --theme-*) consistent across all three
  themes; intersection logic unit-tested (polygon/polygon,
  point/polygon, disjoint).

Est. PR size: ~800–1000 substantive lines excluding tests (~250
Need/Hazard rewire + ~350 Event + ~250 geocode) — at the agreed upper
bound. Pre-agreed split seam if it overruns: the geocode facet lifts out
cleanly as a follow-up PR (the predicate seam ships regardless).

Implementation sketch — written as of 61849d9d on 2026-07-23; rough guide, re-verify against latest

• Replace lib/utils/matchLayers.ts usage with engine calls; retire it
  and the hardcoded options arrays from config.json themes.
• Rail (LayerFilterThemes) untouched — already pulls themes over the bus
  (layerFilter:getThemes).
• Listed writes stay in emitFilterChange → layers:setListed
  (landed by the foundation PR), still gated on first interaction.
• Event theme config: catalog: "activations", facets from: "catalog",
  picker facet isEntry: true, year facet derived: "datetimes"
  (SCHEMA.md §5). Results list reuses the existing checkbox-list
  components; picker rows (year chip + title) per the Figma designs the
  current panels already follow.
• Geocode: check how MapControl invokes its geocoder and reuse the
  service/config pattern (possibly extract a Tools/_shared/ helper).
  Nominatim-style polygon_geojson=1 gives admin polygons; fall back to
  bbox. Intersection via turf booleanIntersects (existing dependency),
  computed on selection only. Engine-side it's a predicate facet:
  selection { label, geometry }.
• Do NOT wire defaultOn/view/time even though rows carry the edge
  attributes — the SCHEMA.md §8 decisions gate that.