Add getLayerCapabilities

[ChrisLoer]

🤖 Description

Layers created with createLayersFromGeoJson hold their whole GeoJSON collection in the browser, so they support a different subset of the SDK than layers hosted by Felt: no server-side query endpoint, no server-computed statistics, no server persistence — but the full feature collection is available locally. felt/felt PRs #21742, #21743 and #21744 make the unsupported methods fail loudly and implement many of the ones that can be served locally.

What's still missing is discoverability (felt/felt ENG-526): a host app has no way to ask which semantics apply to a layer, so it has to guess or probe by calling a method and catching the rejection. getLayerCapabilities closes that gap so a caller can branch up front.

API

getLayerCapabilities(layerId: string): Promise<Array<LayerCapability>>;

export type LayerCapability =
  | "serverPersistence" // the layer exists server-side: duplicateLayer, persistence
  | "serverQuery"       // server-side query endpoint: getFeatures search/sort/filter/boundary, setLayerBoundary, data table
  | "serverStats"       // server-computed statistics: precomputed aggregates, jenks breaks
  | "localData";        // the full GeoJSON collection is in the browser: getFeature/getFeatures/getGeoJsonFeature/local stats

Hosted layers report the server* capabilities; layers created with createLayersFromGeoJson report localData. Each token's JSDoc names the methods that need it, and the method's JSDoc notes that calling a method without its capability rejects with an error naming the missing capability.

⚠️ The Felt-side handler lands separately

This PR is the client half only: the message schema, the controller method, the type, docs and the API spec. Felt does not handle getLayerCapabilities yet — calling it against today's app will fail as an unknown message. The handler implementation lands in felt/felt once this change is published to npm, per the usual SDK flow in DEVELOPING.md.

• felt/felt PR: https://github.com/felt/felt/pull/21742
• Plan: https://claude.ai/code/artifact/18d249b8-0101-4679-83ba-3efdfee53e2a

🧪 Test Plan

• npm run check passes locally (client bundle, format, api-extractor + attw, knip, tsc, vitest, docs).
• New tests/layers.test.ts covers the message round-trip: the layer id reaches the handler, the capability array comes back, a non-string layer id is reported as an invalid message and rejects, and a handler error surfaces as a promise rejection.
• etc/js-sdk.api.md and docs/ are regenerated and committed (npm run update-api, npm run build:docs). The api spec diff is large because adding a public type rotates the bundled chunk hash and shifts every mangled export id — the only semantic change is LayerCapability.
• Behavioral verification happens in the felt/felt PR, which adds the handler and the end-to-end test.