Experimental Cesium entity adapter

@honua/sdk-js/scene-workspace exposes an experimental accepted-plan workflow for projecting a canonical Source query into a Cesium EntityCollection:

import { explainQuery } from "@honua/sdk-js/query-planner";
import { mountSourceToCesium } from "@honua/sdk-js/scene-workspace";

const plan = explainQuery({
  descriptor: source.descriptor,
  query: {
    pagination: { limit: 5_000 },
    returnGeometry: true,
    outSr: 4326,
  },
  sourceVersion,
  schemaVersion,
  authorizationScope,
});

const mounted = await mountSourceToCesium(viewer.entities, source, plan, {
  sourceVersion,
  schemaVersion,
  authorizationScope,
  time: { startField: "observed_at", endField: "expires_at" },
  verticalDatum: "ellipsoidal-wgs84",
});

await mounted.refresh();
mounted.dispose();

The core projection (projectSourceToCesium) does not import Cesium. Mounting accepts either a minimal injected Cesium module or async loader; if neither is provided, the optional cesium peer is imported lazily. Importing the entrypoint in Node/SSR therefore does not initialize a browser or WebGL runtime.

Supported slice

• One accepted, remote query step whose executable query exactly matches the canonical IR, with explicit geometry and a positive row limit no greater than maxEntities (10,000 by default).
• Explicit WGS84 output (outSr: 4326). Coordinates are never silently reinterpreted or reprojected.
• Point, single-part line, and single-ring polygon geometries in GeoJSON or common Esri shapes. Finite Z values require an explicit verticalDatum: "ellipsoidal-wgs84"; otherwise the feature is omitted with a stable fidelity diagnostic.
• Stable entity identity from featureIdField or the source primary key.
• Attributes are copied into deeply frozen JSON-like scalar, dense-array, and plain-object snapshots. Dates, class instances, sparse arrays, accessors, and other non-JSON values omit the feature with an unsupported-fidelity diagnostic rather than retaining mutable caller-owned objects.
• Optional offset-bearing ISO instants with at most millisecond precision, or integer Unix epoch-millisecond start/end attributes, mapped to Cesium availability intervals. Ambiguous local-time strings and precision-losing timestamps are omitted with a fidelity diagnostic.
• Serialized snapshot refresh, cancellation checks before renderer mutation, reentrant-disposal guards, rollback attempts, deterministic cleanup, and retryable failed disposal.
• Stable diagnostics for transfer-limited results, source degradation, missing identity, unsupported geometry, invalid time intervals, and snapshot rebuilds.

Unsupported features are omitted with fidelity: "unsupported"; they are not rendered as plausible substitutes. Invalid plans, CRS, limits, and adapter options fail before mounting.

Deliberate non-goals for this slice

This is not completion of issue #395 and is not yet a beta Cesium adapter. Terrain and imagery providers, glTF/models, point clouds and 3D Tiles continue through the existing scene primitive adapter. Multi-part geometry and polygon holes, vertical datum transforms, styling, clustering, streaming/tiled execution, live deltas, camera/selection/filter synchronization, attribution UI, asset authorization/caching, browser examples, and WebGL leak/performance certification remain future work.

Refresh currently declares an entity-snapshot rebuild boundary. Stable IDs are preserved across rebuilds, but this first slice does not claim fine-grained property or sampled-position delta application.