`Fog.SkipBackground` only exempts pixels at cleared depth exactly 1.0 — any real sky dome is fogged flat; flag silently does nothing

Release Type: Official Release
Version: 4.3.0.2507
Platform(s): All backends

Describe the bug
FogEffect.sdsl implements SkipBackground as if (!skipBG || z_b < 1.0) — it exempts only
pixels still holding the cleared depth of exactly 1.0. Any sky drawn as ordinary geometry
writes a depth just under 1.0 (a 2000 m dome under a 2400 m far plane writes ~0.999992), so the
flag silently does nothing and the sky is blended toward the single FogColor like any other
surface. At a plausible ground-fog density (0.00155/m) that is ~95% fog over the entire sky —
we measured clear-sky patches (185,194,213) vs the brightest cloud (186,194,209): four units of
separation across the whole sky. Nothing in the API documents the depth-exact requirement.

To Reproduce

1. Scene with a sky dome mesh (any radius < far plane) + PostProcessingEffects.Fog with
   SkipBackground = true, density high enough to see.
2. The sky fogs to one flat color despite the flag.

Expected behavior
SkipBackground = true exempts the background/sky, or the docs state the depth-exact contract.

Additional context
Fix shapes: compare against a far-plane epsilon (z_b < 1.0 - eps); or expose a distance-based
cutoff (FogEnd/SkipBeyondDistance) — distance-based is what we shipped app-side (custom
fog shader with a radial-distance exemption) and it also fixes the screen-edge artifacts that a
view-Z exemption produces. At minimum, document that only cleared-depth pixels are exempt.

Start with FogEffect.sdsl and inspect the SkipBackground condition described in the report. Reproduce the issue with a sky dome, then establish whether the fix should use a far-plane epsilon, a distance-based cutoff, or documentation of the cleared-depth contract; done means the chosen behavior is verified with the reported sky scene.