Dispose PlanetariumSkyNode child subtree to release label subscriptions.

[veillette]

Node.dispose() only detaches children (removeAllChildren); it does not
dispose the child subtree. PlanetariumSkyNode.dispose() delegated to
super.dispose() alone, so its label Text nodes (cardinals, star/planet
names) and CelestialLinesNode's derived measure Properties stayed
subscribed to the shared StringManager and model Properties after the sky
node was torn down. A listener-count probe showed three StringManager
string Properties (measureSeparation, sun, north) gaining one listener per
create/dispose cycle and never releasing them.

• PlanetariumSkyNode.dispose(): after detaching the redraw lazyLinks,
  dispose each child tree (the disposeSubtree walk, without re-entering the
  override) so those subscriptions are released. Corrects the dispose
  comment, which claimed super.dispose() recursively disposed children.
• CelestialLinesNode: hold references to the measure label's backing
  DerivedProperty and PatternStringProperty and dispose them (label first,
  so it stops forwarding) in a new dispose() override; Node.dispose() never
  touched these standalone Properties.
• memory-leak.test.ts: lock the fix with a WeakRef regression that fails if
  a cardinal label Text stays retained by the StringManager singleton after
  the sky node is disposed.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_013jBxyCRbyXXCozMaZhqvrd

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