refactor(textures): split upload throttle into processOne / processUntil

[chiefcll]

What changed

Replaces the single CoreTextureManager.processSome(maxProcessingTime) — a sliding, numImageWorkers-sized prefetch window plus a re-queue-on-timeout pass — with two focused methods:

Method	Used when	Behavior
processOne()	animating	upload a single queued texture, then stop
processUntil(maxProcessingTime)	idle	upload serially until the per-frame time budget is exhausted

Shared per-texture work (dead-check → getTextureData guard → upload → try/catch) lives in a private uploadQueued. isTextureDead is a method (not an inline check) so TypeScript doesn't narrow state across the awaits.

Stage picks one based on animation state; the pre-init drain in CoreTextureManager now calls processUntil(Infinity).

Why

The prefetch window was effectively dead weight: loadTexture already awaits getTextureData before enqueuing image textures, so a queued texture is already decoded — the prefetch's getTextureData calls resolved immediately, and the time budget really only bounds GPU upload time. The re-queue-on-timeout pass existed only to recover decodes cut off mid-flight by that budget, which no longer happens. Removing both leaves a straightforward serial drain that's far easier to reason about.

Behavior change

• While animating: exactly one texture is uploaded per frame (previously: as many as fit in half the time budget). This keeps upload cost off the animation frame more predictably.
• Idle: unchanged in spirit — time-budgeted serial uploads, governed by the existing textureProcessingTimeLimit setting (default 10, untouched here).
• Dead (failed/freed) textures that ended up in the queue are skipped for free. Note removeTextureFromQueue still has no callers, so a texture freed after enqueue stays in the queue until drained — uploadQueued discards it.

Testing

• tsc --build clean; 243/243 unit tests pass.
• Browser stress-images (1000 picsum images, idle path): full grid renders and the queue drains to a stable state; residual white squares are confirmed picsum-404 IDs, not stalled uploads. The animating processOne reuses the same verified uploadQueued.

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