Evict one texture unit (LRU) instead of wiping every assignment on exhaustion

[obiot]

Split out of #1584 as a self-contained mitigation.

Current state

packages/melonjs/src/video/texture/cache.js:78 — when no texture unit is free:

// No units available — flush the current batch and reset assignments
if (this.renderer.currentBatcher) {
    this.renderer.currentBatcher.flush();
}
this.units.clear();
this.usedUnits.clear();
...
emit(GPU_TEXTURE_CACHE_RESET);

The eviction policy is evict everything. One texture too many discards all N assignments, and the emitted event makes every batcher drop its cached bindings as well — _onTextureCacheReset() (webgl/batchers/material_batcher.js:100) zeroes boundTextures wholesale. resetUnitAssignments() (cache.js:129) is a second entry point to the same wipe.

The flush itself is unavoidable: quads already in the pending batch reference unit indices, so those vertices must be drawn before any unit is reassigned. Discarding the other N-1 assignments is not.

Proposal

Flush, then evict a single least-recently-used unit and keep the rest. The event carries which unit moved, so a batcher forgets one entry instead of its whole binding table.

	now	after
flushes per overflow	1	1 (unchanged)
textures re-bound per overflow	N	1
few hot textures, occasional cold ones	thrashes	hot ones stay resident
strict round-robin over N+1 textures	thrashes	still flushes each time, re-binds 1

Never worse than today, and in the common case — a handful of textures carrying most draws, plus a tail of occasional ones — it turns the cliff into a slope.

LRU is defeated by strict round-robin, which evicts precisely the texture needed next, every time. That is expected and accepted here: the flush count in that case is already one per texture and this change does not raise it, while still cutting the re-bind count. Removing the round-robin case structurally is #1584's job, not this ticket's.

Scope

• texture/cache.js — per-unit last-use tracking and the evict-one path, in both allocateTextureUnit and resetUnitAssignments
• system/event.ts — the reset event carries a unit index (a full reset stays expressible, for context loss)
• webgl/batchers/material_batcher.js — _onTextureCacheReset(unit) drops one entry; the lit subclass override that also forgets paired normal-map units needs the same narrowing
• webgpu/texture/store.js subscribes to the same event (:39) — its quad path uses segments rather than units, so confirm whether it needs the narrowed form or is unaffected

Testing

Victim selection and the resulting assignment map are pure logic — no GPU required. Given a sequence of texture requests, assert which unit is evicted and that the survivors keep their assignments. Plus:

• pixel readback that the texture set still renders correctly across an overflow, via the existing gl.readPixels idiom
• with the counters from Texture-set overflow thrashes both backends: replace the slot ladder with texture arrays #1584: re-binds per overflow drop from N to 1, while flush count is unchanged
• a regression case pinning that a batcher does not drop unrelated bindings when one unit is reassigned

Related

• Texture-set overflow thrashes both backends: replace the slot ladder with texture arrays #1584 — the structural fix (texture arrays); this is the mitigation that stays useful whatever happens there
• raising the hardcoded batch limit — an independent mitigation, composes with this one