perf(flights): skip dead-reckoning contacts that stay behind the horizon

The fleet tick dead-reckoned, floored and distance-tested every contact on
every ~80 ms tick, then hid the ones behind the horizon. The worldwide feed
carries ~13k contacts; zoomed in, well under 1% are on screen, so nearly all
of that work produced a hidden billboard.

A contact that is already hidden and still behind the horizon at its last
position now re-reckons only on its turn in a staggered one-in-12-ticks
rotation (about once a second, HIDDEN_CONTACT_DR_TICKS). Anything that
position puts in view, and any contact with a fleet model, takes the full
pass immediately, so on-screen behaviour is unchanged. Hidden-inclusive
proximity queries (getNearby includeHidden, getAllPositions) read positions
at most about a second old, well inside the 30 s poll's uncertainty.

Measured in Chrome on a Radeon 780M, ~12.9k contacts, 56 on screen over
Colombia at 4 km, same view before and after:
  fleet tick   7.66 -> 2.03 ms/frame, longest tick 50.9 -> ~16 ms
  frame        20.6 -> 14.0 ms, 42.9 -> 57.8 FPS
Teleporting the camera to Madrid showed 975 contacts within 100 ms.

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

src/layers/flights/rendering.js

@@ -33,6 +33,8 @@ import {
   COURSE_SLEW_DT_MAX_SEC,
   ROTATION_REFRESH_MS,
   COURSE_MAX_DPS,
+  HIDDEN_CONTACT_DR_TICKS,
+  hiddenContactDrPhase,
 } from './policy.js';
 
 export function createRendering({
@@ -811,6 +813,12 @@ export function createRendering({
     }
   }
 
+  /** The contact's hidden-rotation slot, cached on its billboard. */
+  function _hiddenDrPhase(bb, icao24) {
+    bb._gevHiddenDrPhase ??= hiddenContactDrPhase(icao24);
+    return bb._gevHiddenDrPhase;
+  }
+
   function _fleetTick() {
     if (
       !flightState._viewer ||
@@ -931,9 +939,25 @@ export function createRendering({
       for (const icao of toRelease) _releaseModel(icao);
     }
 
+    const hiddenDrPhase = flightState._fleetTickSeq++ % HIDDEN_CONTACT_DR_TICKS;
+
     for (const [icao24, bb] of flightState._billboards) {
       if (icao24 === flightState._trackedIcao) continue; // tracked entity owns its own motion
 
+      // A contact still behind the horizon at its last position would only be
+      // re-reckoned to be hidden again, so off its turn it keeps that position
+      // (see HIDDEN_CONTACT_DR_TICKS). Anything the stale position puts in view,
+      // and any contact with a fleet model, takes the full pass below at once.
+      if (
+        !bb.show &&
+        _hiddenDrPhase(bb, icao24) !== hiddenDrPhase &&
+        !flightState._models.has(icao24) &&
+        !occluder.isPointVisible(
+          flightState._cullPositions.get(icao24) || bb.position,
+        )
+      )
+        continue;
+
       const info = flightState.records.data.get(icao24);
 
       const dr = parts.motion._deadReckon(icao24, flightState._scratchFleetPos);