feat(director): add bounded scene data packs and placement

docs/DIRECTOR-DATA-PACKS.md

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+# Director data packs
+
+Version-5 scene documents can declare `dataPacks` on a scene and select them by
+`dataPackIds` on each shot. Import validates declarations without loading assets.
+Loading, seeking or playing a shot acquires only its selected packs. A shot with
+no pack IDs clears the preceding shot's packs. Existing scene recipes and assets
+keep their current paths, notices and behavior.
+
+A pack manifest is version 1 and requires an ID, format, source, attribution and
+placement. Sources describe storage; placement describes geography. Source names
+refer to trusted application registrations, never scripts to import. A document
+cannot supply request headers, keys, cookies, endpoints or executable handlers.
+
+```json
+{
+  "version": 5,
+  "scenes": [{
+    "id": "example",
+    "title": "My geographic notes",
+    "dataPacks": [{
+      "id": "notes",
+      "version": 1,
+      "format": "geojson",
+      "source": { "adapter": "assets", "path": "my-scene/notes.geojson" },
+      "attribution": { "text": "Your name", "license": "Your asset's actual license" },
+      "placement": { "altitudeReference": "ellipsoid" }
+    }],
+    "shots": [{
+      "id": "overview",
+      "camera": { "lat": 30.2672, "lon": -97.742, "alt": 2600, "pitch": -70 },
+      "durationSec": 4,
+      "holdSec": 2,
+      "dataPackIds": ["notes"]
+    }]
+  }]
+}
+```
+
+For the standalone application, put that asset at
+`public/scene-assets/my-scene/notes.geojson`. The registered `assets` source has
+an explicit `/scene-assets/` base on the application's origin. Importing a file
+from disk does not grant access to adjacent local files or infer a remote base.
+Relative paths cannot contain traversal, query strings, fragments, URL syntax,
+percent escapes or empty segments. Asset directories are served as static files;
+this is not a new arbitrary-URL proxy.
+
+## Formats and placement
+
+- **`geojson`:** a UTF-8 `FeatureCollection`, served as `application/geo+json` or
+  `application/json`. Each feature needs a unique string `id`. Point, LineString
+  and Polygon (including holes) are supported. Coordinates are WGS84
+  `[longitude, latitude, ellipsoidHeightMeters]`; missing height means zero,
+  which can be underground. Supply appropriate heights. Geometry properties
+  are ignored: they cannot select external icons, HTML, styling or scripts.
+  Placement is `{ "altitudeReference": "ellipsoid" }`.
+- **`image`:** PNG served as `image/png`, at most 4096×4096 pixels, checked before
+  decoding. Placement is `{ "bounds": [west, south, east, north], "height": 300,
+  "altitudeReference": "ellipsoid" }`. Bounds must increase and cannot cross
+  the dateline. The image is a horizontal geographic rectangle at that height.
+- **`media`:** MP4/WebM video or MPEG/Ogg/WAV/WebM audio, with the matching standard
+  MIME type. Placement is `{ "anchorId": "media-location" }`, referring to a
+  scene-local anchor with explicit ellipsoid height. A labelled marker locates
+  the media; an attribution card provides native, user-operated playback
+  controls. Media never autoplays. Codec support depends on the browser. D5 does
+  not synchronize media time with scene time or embed third-party players.
+
+Pack attribution always includes nonempty `text` and `license`; an optional
+HTTPS `url` opens as a source link with no opener/referrer. Importing or sharing
+an asset never changes its license. D5 adds no third-party content and preserves
+all existing Nepal content and contributor notices.
+
+Optional `byteLength` specifies the exact expected byte count; optional `sha256`
+is a lowercase hexadecimal SHA-256 digest checked before rendering. There are at
+most 8 packs per scene, 8 MiB per asset, and 32 MiB of selected assets per shot.
+GeoJSON additionally allows at most 2,000 features and 50,000 positions.
+Transforms, terrain-relative placement, terrain collision avoidance, arbitrary
+GeoJSON styles, archives and local filesystem bundles are not implemented.
+
+## Loading and resource ownership
+
+The `director` package export provides `createAssetDirectorySource` and
+`createDataPackSession`. The former receives an explicit trusted HTTP(S)
+directory `baseUrl` and optional `fetchImpl`; it confines every relative asset
+path to that directory. Cross-origin directories require CORS. Requests omit
+credentials and referrers, reject redirects, and use `cache: 'no-store'`.
+There is no persistent pack cache, offline fallback or silent substitute. A
+missing, invalid, oversized or integrity-mismatched asset fails the shot's pack
+load, removes partial results and reports a stable error without exposing a URL.
+Successful loading leaves the attribution cards visible with the assets.
+
+`createApplicationTools({ sceneDataPacks: { sources } })` and
+`new SceneDirector(viewer, style, data, { dataPacks: { sources } })` accept a
+registry of source functions. Each receives `{ path, signal, maxBytes }` and
+returns `{ bytes: Uint8Array, mimeType }`. Custom transports must honor the
+signal, bound reads and return the real MIME type. They remain application code,
+not scene document fields. Without registrations, referenced sources fail closed.
+
+The portable session receives format adapters separately. Each adapter receives
+`{ pack, asset, anchors, signal }` and returns a resource with an idempotent
+`dispose()`. Core's scene composition supplies Cesium geometry/image rendering
+and text-only attribution/native media controls. The session applies a 15-second
+load deadline and rejects late results, even when an adapter ignores abort.
+
+Stop, replacement, next-shot selection, scene release and destruction cancel
+pending work, remove owned entities/cards, stop media and revoke object URLs.
+Run completion releases packs with its run signal. Direct LOAD and seek retain
+packs until the next operation or Stop. Resources are reacquired per shot;
+there are no disabled pack refresh loops. `getDataPackState()` returns copied
+status and resource counts for lifecycle diagnostics.
+
+Run `node scripts/qa-director-packs.mjs` against a credentialed local server to
+exercise the real import/load/seek/Stop path with self-authored synthetic assets.
+This stage provides the manifest/loading boundary; interactive actions and
+visual authoring follow separately.