Cesium: make plugins and on-map controls engine-aware

[giswqs]

Part of #2259.

Problem

GeoLibreAppAPI exposes getMap(): MapLibreMap | null and addMapControl(control: IControl). Of 141 modules under packages/plugins/src/plugins/, 62 touch the MapLibre map directly (getMap(), addMapControl, an IControl implementation, or a maplibre-gl import): 13 call getMap() and 23 register an IControl.

On the globe getMap() returns null and addMapControl returns false, so those plugins activate and then do nothing visible — the worst failure mode, because the toggle reports success.

Three tiers

Only one of them needs substantial work.

Tier 1 — already engine-neutral

Catalog and service browsers (STAC, Planetary Computer, NASA Earthdata, Source Coop, ArcGIS Hub, Open Data Catalogs, EnviroAtlas, FEMA WMS, …) only call app.addTileLayer / addWmtsLayer / addGeoJsonLayer, which are pure store writes in createAppAPI. These work on the globe today once #2261 covers the layer kind they produce.

• Audit which plugins are genuinely store-only and un-gate them.

Tier 2 — DOM controls (23 modules)

Layer control, basemap control, components (legend / colorbar / HTML), time slider, swipe, … A MapLibre IControl is just onAdd(map) → HTMLElement. Two pieces make these portable:

• Render the four .maplibregl-ctrl-{top,bottom}-{left,right} corner containers over the Cesium canvas, so control DOM lands in the right place and the scoped overrides in apps/geolibre-desktop/src/index.css keep working unchanged.
• Pass onAdd a map facade implementing the subset these controls actually use — getContainer, getCanvas, on / off / once, project / unproject, getCenter / getZoom / getBearing / getPitch, getBounds, flyTo / easeTo / jumpTo. All have exact Cesium equivalents.
• The facade must fail loudly on addSource / addLayer / setPaintProperty rather than no-op. That is the honest boundary between tier 2 and tier 3, and a silent no-op there reproduces the exact bug this issue is about.

Tier 3 — plugins that paint through MapLibre

Graticule, H3 / S2 / geohash / A5 / DGGS grids, annotations, geo-editor, effects, clouds, route animation. These need real Cesium implementations (GeoJsonDataSource, Entity, PolylineCollection) or they stay MapLibre-only.

• Implement per plugin, as demand dictates. Not a blocker for the rest.

Declare engine support

• Add engines?: MapRendererKind[] to the plugin manifest, defaulting to ["maplibre"].
• Gate the Plugins menu on it, so a plugin that cannot run on the current engine is greyed out honestly instead of toggling into nothing.
• Document it in docs/plugin-api.md — external plugin authors need a declaration point, and this is also the compatibility contract for third-party plugins once a second engine is real.

Depends on

The engines field and the tier 1 audit are independent and cheap — worth doing early. Tier 2's facade is easiest once #2260 has defined MapEngine, since the facade is a thin MapLibre-shaped adapter over the same CesiumEngine.

#2263 builds that facade against two real consumers (Search Places and the Basemaps plugin) and should land before the rest of tier 2 — with the seam in place, the remaining 21 IControl plugins become an audit rather than a design problem.

[giswqs]

Tier 2's hard part is done. #2266 landed CesiumControlHost — the four .maplibregl-ctrl-* corner containers over the Cesium canvas plus a MapLibre-shaped facade covering getContainer, getCanvas, on/off/once, getCenter/getZoom/getBearing/getPitch, jumpTo/flyTo, and setStyle (routed to the store), with the style-spec methods throwing deliberately and addControl/removeControl catching so a throwing control cannot take down plugin activation.

So the remaining tier 2 work is now the audit rather than the design: walk the ~22 other IControl plugins and check which ones the facade already satisfies. The engines?: MapRendererKind[] manifest field and the tier 1 audit are still open and still cheap.