Draw the last native world labels through the label overlay

The Directions A and B letters and a data pack's media label were the two
Cesium text labels left, kept by an allow-list in noCesiumLabels.test.mjs.
Both now publish entries to the shared world-overlay host, and the
allow-list is gone, so any Cesium label in src is a test failure.

Directions publishes the letters to the directions-markers source; the
markers keep only their points. A clicked endpoint's letter hangs over the
surface that was clicked. One placed without a click stays hidden until its
ground-floor cell resolves, then requests a render, so a still camera shows
it; a cell that settles with no height falls back to the ellipsoid. Each
position is computed once.

A pack resource handle gains label(entry). The application passes its
shared overlay host to SceneDirector through dataPacks.overlayHost; without
one a media pack fails to load rather than drawing a native label. Each
labelled resource takes a source from a pool and returns it on dispose, as
the host never removes a source, so two presentations of one pack never
clear each other's labels and repeat loads add no sources. A label written
after dispose is ignored. The media label is protected, so it always draws
as before.

Live (Denver, keyless globe): a clicked A and a programmatic B both anchor
on their dots over 1.58 km ground. Documented in docs/CURRENT-STATE.md and
docs/DIRECTOR-DATA-PACKS.md.

docs/DIRECTOR-DATA-PACKS.md

@@ -95,6 +95,11 @@ 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.
+A standalone `SceneDirector` also passes `dataPacks.overlayHost`, the
+world-overlay host that draws pack world text such as a media pack's label (an
+object with `setEntries(sourceId, entries)` and `clearSource(sourceId)`);
+without it a media pack fails to load. `createApplicationTools` passes the
+application's shared host.
 
 The portable session receives format adapters separately. Each adapter receives
 `{ pack, asset, anchors, signal }` and returns a resource with an idempotent