fix(render): keep Cesium's model atmosphere off Apple Metal

iPad and iPhone show "An error occurred while rendering. Rendering has
stopped." on load. Cesium's AtmosphereStageVS calls
czm_computeGroundAtmosphereScattering with its `out` parameters bound
directly to varyings:

    czm_computeGroundAtmosphereScattering(
        v_positionWC, lightDirection,
        v_atmosphereRayleighColor, v_atmosphereMieColor,
        v_atmosphereOpacity);

ANGLE's Metal backend wraps each `out` argument in an ANGLE_out(...)
helper taking `thread T &`, but vertex-output members live in
`__metal_generic` and cannot bind to `thread`, so the program fails to
LINK and Cesium tears down the render loop.

This is the only shader in Cesium that binds varyings to `out` params --
SkyAtmosphere (czm_computeScattering) and the ground-atmosphere fragment
path both route through locals, so they are unaffected and stay on.

ModelSceneGraph.configurePipeline attaches the stage under a single
condition, `fog.enabled && fog.renderable`, so clearing `fog.renderable`
keeps the broken program from ever being built.

`renderable` rather than `enabled` is deliberate: Fog.update returns
early when `enabled` is false and forfeits the density that drives 3D
Tiles screen-space-error scaling, and visualSettings saves/restores
`fog.enabled` around IR boost, which would resurrect the broken shader
mid-session.

Detection is a WebGL2 link probe of the same out-param/varying pattern,
so a future driver fix silently restores the effect with no code change,
OR'd with iOS/iPadOS platform detection as a backstop for when no probe
context can be created. iPadOS desktop mode is identified by the
Macintosh UA token plus maxTouchPoints > 1, since real Macs report zero
and navigator.platform is deprecated.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01HoRD3yHVgfTHeikTmsUoHf

scripts/package-boundaries.json

@@ -20,7 +20,7 @@
   },
   "viewer": {
     "exports": ["./application/viewer"],
-    "modules": ["src/app/viewer.js"],
+    "modules": ["src/app/viewer.js", "src/app/atmosphereCompat.js"],
     "external": ["cesium"]
   },
   "vite-build": {
@@ -1287,6 +1287,7 @@
       "src/app/surfaceServices.js",
       "src/app/tools.js",
       "src/app/viewer.js",
+      "src/app/atmosphereCompat.js",
       "src/bloom.js",
       "src/camera.js",
       "src/cameraVerbs.js",