refactor(flights): Cesium rendering behind a renderer interface

Task 10a. The flights logic no longer imports Cesium: every Cesium object it
built moved one for one into the shared aircraft renderer
(src/layers/aircraft/render/cesium.js; contract in render/contract.js), with
the flights' parameters in src/layers/flights/render/cesium.js, registered
for 'cesium' in the renderer registry and created at init(viewer, {engine}).

- Fleet: BillboardCollection + sprite-order slot; sprites stay Billboard
  handles (field vocabulary documented for the Navara renderer).
- Models: fleet/tracked/preload glTF loads (asynchronous:false, MIX tint, IR
  boost as `unlit` → the UNLIT CustomShader), the model collection, setPose
  (same HPR frame with airplane.glb's 180° offset).
- Tracked: the ENU billboard Entity with its position/colour/rotation
  callbacks, viewFrom, followed through viewer.trackedEntity + the one exact
  tracked frame (kept as is; engine.camera.follow on Cesium would also end
  the follow on any camera command, a behaviour change).
- Trail: the entity trail (data/trailRenderer, unchanged: its Primitive
  predecessor failed below the mesh) and the live head segment.
- View: camera/scene reads, frame number, frustum and horizon tests,
  projection, render requests, frame events, evidence render-loop hold.
- Clicks and Escape go through the engine's selection dispatcher (the
  ScreenSpaceEventHandler and keydown listener are gone); the cross-layer
  untrack watches the renderer's follow.
- Colours are engine-neutral Rgba (src/layers/shared/color.js), bit-equal to
  the Cesium.Color constants they replace; NearFarScalar → nearFarScale.
- Cesium math in the logic → src/geo.
- check-import-directions: cesium only in render/cesium*.js and three/Navara
  only in render/navara*.js for flights, aircraft and shared; a layer's
  Navara renderer only through navaraMode's import().

Source-text tests updated with the moved code, same meaning:
- readLayerSource reads a layer's render/*.js and, for the aircraft layers,
  aircraft/render/*.js (GLB-load, tracked-entity, CallbackProperty checks).
- modelScale.test: _normalBillboardScaleByDistance reads nearFarScale(...).
- spriteOrder.test: flights' enable calls fleet.restoreOrderOnEnable(); the
  renderer wires it to restoreSpriteOrderOnEnable('flights', viewer).
- trackingClickGesture.test: the flights gesture rules are the dispatcher's
  (selection gesture before select, click classifier before the empty-space
  clear); flights answers the empty click with a user-origin clear.
- militaryAwareness.test: flights' hasContact guard reads fleet.visible().
- ownership.test: the services fixture names selection and renderer.

New tests: color, the Cesium aircraft renderer (contract, collections, pose,
loads, frustum/horizon, tracked entity, follow, trail head, adopted
collections), the flights logic over a plain-data renderer and a fake engine
(fleet, click-select, no-op on the tracked visual, empty-click release), the
boundary rule.

W1 replay vs 7c7422e2: counts, known objects, screen points (0 px), tracking
poses (0 m), requests and shots (same sha256) identical. Main chunk
2,890,982 B (+6.2 KB, +0.21 %: the renderer indirection).

scripts/package-boundaries.json

@@ -896,7 +896,8 @@
       "src/geo/developerError.js",
       "src/time/clock.js",
       "src/globe/contract.js",
-      "src/globe/cesium/engineFromViewer.js"
+      "src/globe/cesium/engineFromViewer.js",
+      "src/layers/shared/color.js"
     ],
     "external": ["cesium"]
   },
@@ -2587,7 +2588,11 @@
       "src/globe/surfaceDetail.js",
       "src/globe/navara/followGestures.js",
       "src/app/layers/directory.js",
-      "src/app/layers/renderers.js"
+      "src/app/layers/renderers.js",
+      "src/layers/aircraft/render/cesium.js",
+      "src/layers/flights/render/cesium.js",
+      "src/data/selectionDispatcher.js",
+      "src/layers/shared/color.js"
     ],
     "external": [
       "hls.js",
@@ -3158,7 +3163,11 @@
       "src/globe/follow.js",
       "src/globe/cesium/follow.js",
       "src/app/layers/directory.js",
-      "src/app/layers/renderers.js"
+      "src/app/layers/renderers.js",
+      "src/layers/aircraft/render/cesium.js",
+      "src/data/selectionDispatcher.js",
+      "src/layers/shared/color.js",
+      "src/layers/flights/render/cesium.js"
     ],
     "external": [
       "hls.js",