Return null on asset cache miss instead of throwing

[Copilot]

AssetCache.get() previously threw on a cache miss, forcing callers to either pre-load assets or catch errors. It now returns null | T, aligning with standard cache lookup semantics.

Changes

• AssetCache interface — get return type changed from T to T | null
• ImageCache.get() — returns null on miss instead of throwing
• ImageCache.getOrLoad() — guards against the (theoretically unreachable) null after load() to satisfy the type system
• image-cache.test.ts — updated cache-miss test to assert null instead of a thrown error

Before / After

// Before
imageCache.get('missing.png'); // throws Error

// After
imageCache.get('missing.png'); // returns null

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