feat(sky): dim/brighten the moon highlight through its phase

[soloturn]

Companion to a MovingBlocks/Terasology engine PR adding CelestialSystem/BackdropProvider#getMoonPhase() for MovingBlocks/Terasology#94 - a hook for basing the moon's phase on the game's day counter.

What changed

sky_frag.glsl's moon highlight (the soft glow rendered opposite the sun once it's below the horizon - there's no separate textured moon disc) previously used a hardcoded MOON_HIGHLIGHT_INTENSITY_FACTOR of 1.0, so it was always full brightness regardless of what phase the moon should be in. Replaced that #define with a moonPhaseIntensity uniform, set in BackdropNode from 1.0 - abs(backdropProvider.getMoonPhase() * 2.0 - 1.0) - a triangle wave that's 0 at new moon (phase 0 or 1) and 1 at full moon (phase 0.5), folding smoothly between. No new textures or art assets needed - this reuses the existing highlight rendering, just no longer holds its peak brightness constant.

⚠️ Please verify live before merging

Compiles against a local engine checkout carrying the paired getMoonPhase() addition. No GL context available here to render and confirm visually - the shader/uniform change is small and mirrors the existing sunExponent/skySettings wiring pattern already used a few lines above it in BackdropNode, but this genuinely needs an eyeball check in-game across a few in-game days to confirm the fade reads correctly (e.g. via the sun-halting debug command to fast-check specific phases).

Companion PR: MovingBlocks/Terasology#5387

[coderabbitai]

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