perf + lighting: PCFSoft shadows, fill light, tighter frustum, quality-tiered shadow maps

[Copilot]

The Three.js scene had a fixed-resolution shadow map regardless of quality tier, an oversized shadow frustum wasting texel density, only a single key light (flat-looking scene), and a per-frame fog/background update running unconditionally every tick.

Performance

• Remove WorldBackground.useFrame — was calling scene.background.set() + fog updates every frame; the declarative JSX and existing useEffect already cover this
• Quality-tiered shadow maps (performance.js) — shadowMapSize 512 / 1024 / 2048 for low / medium / high; was hard-coded to 1024 for all tiers
• Tighter shadow frustum — ±7/±8 → ±5/±6 world units; fully covers the play area while reducing projected area ~40%, giving ~1.7× better shadow-texel density at the same resolution
• Ball sphere segments — 24×18 → 18×12; saves ~600 tris/frame, imperceptible at gameplay distances

Shadow quality

• PCFSoftShadowMap on the Canvas — replaces default hard-edge PCF with soft penumbrae
• shadow-radius={2} on the key light — adds PCFSoft blur for smoother edges

Lighting

• Fill/rim light — cool-tinted (#b8cfe8) directional from [-3, 5, -4] at intensity 0.55, opposite the warm key at [4.5, 7.5, 5.5]; gives side-face separation on paddles/net that was completely absent before; subtly reacts to arenaFx.heat and flash
• Exposed as TUNING.lighting.fill / fillColor with matching debug-panel controls in the LightingPost folder