fix(layers): release bundled GeoJSON entities on toggle-off instead of hiding them

Toggling datacenters, dams, or submarine cables off only set `show = false`
on their data sources. Cesium's DataSourceDisplay still walks every
visualizer of a hidden source each frame, and the ~11.5k hidden entities
(hundreds of MB) stayed resident, so the whole scene rendered ~2x slower
for the rest of the session after a single on/off round trip (owner field
test 2026-09-13).

disable() now removes the data sources from the viewer and drops the built
records; the parsed dataset (NDJSON features / TeleGeography JSON) is cached
so enable() rebuilds without a refetch or re-parse. A disable that lands
while a load is in flight releases the finished build instead of parking it
hidden. destroy() clears the cache.

Measured with a headful Chrome harness (flights+military+satellites on,
camera rotating, 250-frame windows), before -> after:
- entities resident after OFF: 11,537 -> 0
- DataSourceDisplay.update per frame after OFF: 1.09 ms -> 0.02 ms
- scene.render per frame after OFF: 13.9 ms (control 6.6 ms) -> 5.2 ms
  (fresh baseline 5.0 ms in the same run)
- JS heap 30 s after OFF: 1.6 GB and climbing -> 342 MB after GC

Tests: the GeoJSON lifecycle and cables suites assert disable leaves zero
sources/contexts and re-enable rebuilds with fetch called once.

src/data/localGeojsonLifecycle.test.mjs

@@ -54,7 +54,8 @@ test('destroy removes context records and permits a fresh replacement', async t
   const oldEntity = [...getContextStore().entities.values()][0].entity;
   assert.equal(getContextStore().entities.size, 1);
   env.layer.disable(env.viewer);
-  assert.equal(getContextStore().entities.size, 1, 'disabled cached source retains its records');
+  assert.equal(getContextStore().entities.size, 0, 'disable releases its records with the source');
+  assert.equal(env.sources.size, 0, 'disable removes the built source from the scene');
   env.layer.destroy(env.viewer);
   assert.equal(getContextStore().entities.size, 0);
   assert.equal(env.sources.size, 0);
@@ -126,17 +127,41 @@ test('concurrent enable and disable/re-enable share one pending dataset load', a
   assert.equal([...env.sources][0].show, true);
 });
 
-test('disable during loading keeps the completed source hidden until re-enabled', async t => {
+test('disable during loading releases the completed source; re-enable rebuilds without refetching', async t => {
   const env = harness(t);
   const release = deferred();
-  t.mock.method(globalThis, 'fetch', async () => { await release.promise; return response(); });
+  const fetchMock = t.mock.method(globalThis, 'fetch', async () => { await release.promise; return response(); });
   const loading = env.layer.enable(env.viewer);
   env.layer.disable(env.viewer);
   release.resolve();
   await loading;
-  assert.equal([...env.sources][0].show, false);
+  assert.equal(env.sources.size, 0, 'a build that finishes after disable() is released, not parked hidden');
   assert.equal(env.listeners.size, 0);
+  assert.equal(getContextStore().entities.size, 0);
   await env.layer.enable(env.viewer);
-  assert.equal([...env.sources][0].show, true);
   assert.equal(env.sources.size, 1);
+  assert.equal([...env.sources][0].show, true);
+  assert.equal(fetchMock.mock.callCount(), 1, 'the parsed dataset is cached across the toggle');
+});
+
+test('a toggle-off frees the entities and a toggle-on rebuilds them from the cached dataset', async t => {
+  const env = harness(t);
+  const fetchMock = t.mock.method(globalThis, 'fetch', async () => response());
+  await env.layer.enable(env.viewer);
+  const first = [...env.sources][0];
+  assert.equal(first.entities.values.length, 1);
+  assert.equal(getContextStore().entities.size, 1);
+  env.layer.disable(env.viewer);
+  assert.equal(env.sources.size, 0, 'no data source stays registered while the layer is off');
+  assert.equal(getContextStore().entities.size, 0);
+  assert.equal(env.layer.getLodDiagnostics().total, 0);
+  assert.equal(env.layer.getStats().count, 1, 'the row keeps reporting the dataset size');
+  await env.layer.enable(env.viewer);
+  const second = [...env.sources][0];
+  assert.notEqual(second, first, 're-enable builds a fresh source');
+  assert.equal(second.show, true);
+  assert.equal(second.entities.values.length, 1);
+  assert.equal(getContextStore().entities.size, 1);
+  assert.equal(fetchMock.mock.callCount(), 1, 'no second fetch for a bundled dataset');
+  assert.equal(env.layer.getLodDiagnostics().total, 1);
 });