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/policy.js

@@ -200,6 +200,37 @@ export const FLEET_DR_INTERVAL_MS = 80;
 
 export const ROTATION_REFRESH_MS = 1000;
 
+/**
+ * @constant {number} Dead-reckoning cadence (ms) for contacts hidden behind the
+ * horizon. The worldwide feed carries ~13k contacts and a zoomed-in view shows
+ * well under 1% of them; the rest still need positions for hidden-inclusive
+ * proximity queries, but ~1 s old (≈250 m for an airliner) is well inside the
+ * 30 s poll's own uncertainty. Each hidden contact is re-reckoned on one tick in
+ * every HIDDEN_CONTACT_DR_TICKS, staggered by id so no tick carries them all.
+ */
+
+export const HIDDEN_CONTACT_DR_INTERVAL_MS = 1000;
+
+export const HIDDEN_CONTACT_DR_TICKS = Math.max(
+  1,
+  Math.round(HIDDEN_CONTACT_DR_INTERVAL_MS / FLEET_DR_INTERVAL_MS),
+);
+
+/**
+ * Stable slot in [0, HIDDEN_CONTACT_DR_TICKS) for a contact id, so each hidden
+ * contact re-reckons on the same tick of every rotation.
+ * @param {string} id Contact id (ICAO24).
+ * @returns {number}
+ */
+
+export function hiddenContactDrPhase(id) {
+  const text = String(id);
+  let hash = 0;
+  for (let i = 0; i < text.length; i++)
+    hash = (hash * 31 + text.charCodeAt(i)) >>> 0;
+  return hash % HIDDEN_CONTACT_DR_TICKS;
+}
+
 /** Max course slew (deg/s) — well above real turns (≤4°/s), hides fix-boundary
  *  steps. Scaled down toward COURSE_MIN_DPS at low speed (courseSlewCapDps). */