fix(text): bound SDF layout cache eagerly; debug overlay uses SDF

[chiefcll]

The bug

Running the debug overlay over animated text for several minutes on a low-RAM device leaked JS heap until OOM → the renderer's context died → black screen / text stops drawing. Repro: ?test=stress-single-level-text&debug=true&multiplier=10.

Investigation (measured, not guessed)

Sampled performance.memory across controlled variations:

Scenario	heap growth
test, no debug	~0.09 MB/s (flat) — the test itself does not leak
test, debug=true	~0.9 MB/s
contextSpy+fps, no overlay	flat → ruled out the spy/StatTracker
both fixes below	~0.004 MB/s (flat) ✅

The leak only appears with debug=true, i.e. the debug overlay — not the test.

Fixes

1. Debug overlay → SDF text (examples/index.ts) — the dominant leak (~0.9 MB/s)
The overlay's text node specified no font, so it used the Canvas text renderer, which rasterizes a fresh ImageTexture (a full bitmap, ~390 KB at this size) on every text change (CoreTextNode.ts:254) — and the overlay rewrites its text every interval (~2/s). Forcing fontFamily: 'Ubuntu' + textRendererOverride: 'sdf' makes it draw from the shared atlas with no per-update texture allocation.

2. Bound the SDF layout cache eagerly (SdfTextRenderer.ts) — a real renderer bug
The SDF layout cache only evicted on idle (cleanup, by design "so eviction never competes with rendering"). But a continuously animating scene never goes idle, so any app with ever-changing SDF text — a clock, counter, score, or fps readout — grew the cache without bound. Now it's also bounded on insert: a single Map delete on a cache miss (only when new text is laid out), which doesn't compete with steady-state rendering. Idle cleanup is retained for bulk trimming. Updated the cache test to assert the eager-eviction behavior.

Notes

• Fix [fix] [sdf] Sdf text alpha double multiplication #2 changes a documented design choice (idle-only eviction). The per-insert cost is negligible and it closes a genuine long-run leak for changing-text apps; happy to adjust if you'd prefer it gated.
• Fix 🐛 [BUG] - CanvasTextRenderer tints color-emoji glyphs with node.color, crushing native emoji colors #1's root cause was the debug overlay added in feat(examples): live on-screen debug overlay (?debug=true) #75 — this is a follow-up to that.

Testing

• pnpm build, pnpm lint clean; pnpm test 314/314 (rewrote the SDF cache test for eager eviction).
• Browser-verified the original repro: ~0.9 MB/s → flat (~0.004 MB/s) over a multi-minute run.

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