perf(vessels): let the scene idle with AIS vessels enabled

Vessels move only when a poll lands, yet enabling the layer held the
continuous render loop for its lifetime, so a parked vessels-only scene
rendered every frame. The layer now:

- requests a frame for every forced visibility pass (new data, selection);
- leaves one trailing frame when the camera moved inside the 800 ms
  visibility cadence, so the view the camera stopped on is evaluated. It is
  keyed to camera motion alone: the pass's own label publication requests a
  frame, and scheduling on that looped at ~1.2 Hz;
- holds 'ais-vessels' only while focus emphasis is animating.

The 80 ms focus pass, which runs while anything is tracked, also skips hidden
sprites whose focus state is at rest (new spriteFocusAtRest) and whose alpha
is already 1: advancing them changes nothing, and with a 12k worldwide feed
they are most of the fleet.

Measured: a parked vessels-only scene (headless SwiftShader) went from
continuous mode and 28 frames in 5 s to idle and 0; horizon visibility had no
mismatches after the camera moved and settled. The focus pass over 12,000
vessels (11,353 hidden) went from ~4.0 ms to ~1.2 ms.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/data/focusDeemphasis.js

@@ -555,6 +555,21 @@ export function advanceFocusEvidenceNowMs(deltaMs) {
   return _focusEvidenceNowMs;
 }
 
+/**
+ * True when a sprite holds no dimming state, or has settled at full emphasis
+ * with nothing pending. Advancing such a sprite without a screen position (a
+ * hidden sprite) changes neither its state nor its alpha, so a layer pass may
+ * skip it.
+ * @param {object} sprite
+ * @returns {boolean}
+ */
+export function spriteFocusAtRest(sprite) {
+  const state = _spriteStates.get(sprite);
+  return (
+    !state || (state.factor === 1 && state.desired === 1 && !state.wasFarther)
+  );
+}
+
 /** Forget state when a primitive is removed from its owning collection. */
 export function forgetSpriteFocus(sprite) {
   if (sprite) _spriteStates.delete(sprite);