fix(core): release shader value cache & subtexture listeners on destroy

[chiefcll]

Background

While fixing the unbounded SDF text layout cache (#55), I audited the codebase for the same class of leak — things that grow over a long-lived session (embedded/set-top, page never reloaded) as nodes and textures are created/destroyed during navigation. This PR fixes the two HIGH-severity leaks found that occur per-node during normal navigation. (Two MEDIUM findings — colorCache and normalizedMetrics — are noted below as follow-ups.)

Leak 1 — shader value cache never released on node destroy

CoreShaderManager.valuesCache / valuesCacheUsage are keyed by resolved shader props + node width/height. Usage is incremented when values are first computed and only decremented in the shader node's update() when the key changes. The idle cleanup() only evicts entries at usage <= 0, so a destroyed node's last value-cache entry stayed pinned at usage >= 1 forever and could never be reclaimed.

This affects nearly every built-in shader that defines an update (RoundedRectangle, Border, Shadow, the gradients, HolePunch, RadialProgress…), so any shader-styled node leaks an entry on destroy.

Fix: move the held valueKey onto the base CoreShaderNode and add detachNode(), which decrements that key's usage and detaches from the node. CoreNode.destroy() calls it for non-default (per-node) shaders — the shared default shader is skipped. This mirrors the existing prevKey release done on every key change in update().

Leak 2 — SubTexture never unsubscribed from its parent atlas

SubTexture attaches 4 listeners (loading/loaded/failed/freed) to its parent texture, which is typically a long-lived shared atlas (preventCleanup). It had no destroy() override, so every destroyed SubTexture — and everything its handlers captured — was retained by the parent's listener lists for the rest of the session.

Fix: SubTexture.destroy() now off()s the four handlers before super.destroy(); Texture.destroy() additionally calls removeAllListeners() defensively so any texture destroyed while something still holds a listener doesn't retain it.

Testing

• pnpm build clean; full unit suite green; lint clean (lint-staged on commit).
• New tests:
  • CoreShaderNode.test.ts — detachNode() decrements the held key, clears it, is a no-op when no key is held, and doesn't double-decrement.
  • SubTexture.test.ts — destroy() detaches all four parent listeners.
• No visual regression test — no rendering-output change; these are teardown/memory fixes.

Follow-ups (not in this PR)

• colorCache (src/core/lib/colorCache.ts) — unbounded Map<number,string>, Canvas2D path only. Bound with the same idle-LRU pattern as feat(text): bound SDF text layout cache with idle LRU eviction #55 if the Canvas backend is used.
• normalizedMetrics (font handlers) — not cleared on unloadFont; bounded for a fixed font set but grows with fractional/scaled sizes.

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