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

docs/CURRENT-STATE.md

@@ -664,6 +664,24 @@ snapshots, lifecycle actions and row descriptors. Remount and teardown remove
 listeners and row subscriptions; obsolete completions do not repaint old rows.
 The clear control presents busy state while its existing action owns the transaction.
 
+
+## Model atmosphere on Apple Metal
+
+Cesium's per-vertex model atmosphere is kept out of the pipeline on devices
+whose driver cannot link it. `AtmosphereStageVS` binds shader `out` parameters
+directly to varyings, which ANGLE's Metal backend rejects at link time, tearing
+down the render loop on iPadOS and iOS. `ModelSceneGraph.configurePipeline`
+attaches that stage only when `fog.enabled && fog.renderable`, so the viewer
+clears `scene.fog.renderable` and leaves `fog.enabled` — and the fog density
+that drives 3D Tiles screen-space-error scaling — intact. Sky atmosphere and the
+ground-atmosphere fragment path route through locals and are unaffected.
+
+Detection is a WebGL2 link probe of the same out-parameter/varying pattern, so a
+future driver fix restores the effect with no code change; iOS/iPadOS platform
+detection is the backstop for when no probe context can be created. Applied
+during viewer construction, before any tile builds a draw command.
+
+
 ## Map Source control ownership
 
 Map Source controls own chip listeners, source-state subscriptions and selection