feat(sdr): local receivers — browser RTL-SDR, Local ADS-B layer and decoder feeds (#732)

Adds a Local ADS-B layer fed by your own receivers: a browser WebUSB
RTL-SDR (with FM listening) and local 1090/978 MHz decoder feeds
(LOCAL_RECEIVER_FEEDS, local and private addresses only). Local aircraft
draw in magenta with class, silhouettes, 3D models, trails and a position
sanity filter; the click card names the band and input that heard them.

Builds on Sameh Khamis's SDR prototype and the initial GEV port by
Mazeyar Moeini Feizabadi (#134); the decoder-feed design follows the
notes on #57. Closes #57.

Co-authored-by: Sameh Khamis <[email protected]>
Co-authored-by: Mazeyar Moeini Feizabadi <[email protected]>

src/data/modelEligibility.js

@@ -0,0 +1,45 @@
+/**
+ * Shared 3D-model eligibility for the aircraft layers.
+ *
+ * Eligibility is by DISTANCE (the mode's add/keep band) with ON-SCREEN
+ * PRIORITY under the cap, in four visible-first passes so the slots are spent
+ * on what the operator can see:
+ *  1. KEEP on-screen contacts that already have a model (hysteresis for
+ *     visible planes);
+ *  2. ADD on-screen new contacts inside the add radius, nearest first;
+ *  3. KEEP off-screen contacts that already have a model;
+ *  4. ADD off-screen new contacts inside the add radius with leftover slots.
+ * KEEP is split by frustum so an off-screen retained model can never starve
+ * an on-screen contact that wants one.
+ *
+ * Portable: no Cesium, DOM or layer state.
+ */
+
+/**
+ * @param {Array<[string, number, boolean]>} candidates `[id, distanceSq,
+ *   onScreen]` for every contact inside the KEEP radius, sorted nearest first.
+ * @param {object} options
+ * @param {number} options.cap Maximum number of eligible contacts.
+ * @param {number} options.addDistSq Squared ADD radius (m²).
+ * @param {(id: string) => boolean} options.isModeled Whether the contact
+ *   already has a model (live, not merely pending).
+ * @returns {Set<string>} Contacts that may hold a model this tick.
+ */
+export function selectModelEligible(candidates, { cap, addDistSq, isModeled }) {
+  const eligible = new Set();
+  const passes = [
+    (onScreen, modeled) => onScreen && modeled,
+    (onScreen, modeled, inAdd) => onScreen && inAdd,
+    (onScreen, modeled) => !onScreen && modeled,
+    (onScreen, modeled, inAdd) => !onScreen && inAdd,
+  ];
+  for (const admit of passes) {
+    for (const [id, distanceSq, onScreen] of candidates) {
+      if (eligible.size >= cap) return eligible;
+      if (eligible.has(id)) continue;
+      if (admit(onScreen, isModeled(id), distanceSq <= addDistSq))
+        eligible.add(id);
+    }
+  }
+  return eligible;
+}