fix(webgl): release throwaway context in premultiply-alpha probe

[chiefcll]

What

detectPremultiplyAlphaHonored (src/core/lib/validateImageBitmap.ts) creates a temporary WebGL context to test whether createImageBitmap(..., { premultiplyAlpha: 'premultiply' }) is honored by the browser. It deleted the texture and framebuffer it used, but left the WebGL context itself dangling until GC.

This change releases that context immediately via WEBGL_lose_context.loseContext() on the exit path.

Why

Reported on Apollo/Sunrise TV boxes (Chrome 38+):

There are too many active WebGL contexts on this page, the oldest context will be lost
Could not create WebGL Texture   (WebGlCtxTexture.js)
Failed to upload texture: Error: Could not create WebGL Texture   (CoreTextureManager.js)

Embedded TV SoCs cap the number of live WebGL contexts very low. This probe runs in a microtask after the main render context is created, so a leaked probe context is the newest on the page. Holding that extra slot can push the page over the device's context budget, and the browser evicts the oldest context — the live render context. Once that happens, gl.createTexture() returns null and the engine spams Could not create WebGL Texture every requestAnimationFrame.

Don't wait for GC to reclaim the canvas — drop the context the moment the probe is done.

Notes for reviewers

• The loseContext event fires only on the throwaway canvas, so it does not trip the main renderer's webglcontextlost handler in WebGlRenderer.ts.
• This path only runs when the app opts in with premultiplyAlphaHonored: 'auto'. The default is true (no probe), so default configs are unaffected.
• This is the secondary half of the fix. The primary context-leak source for the reported issue is in the consuming @solidtv/solid layer (getWebglSupportedVersions, which re-created a context on every non-default call); that fix ships as a separate PR in that repo.

Tests

• Added getExtension to the mock GL in validateImageBitmap.test.ts and a test asserting loseContext() is called exactly once.
• pnpm vitest run src/core/lib/validateImageBitmap.test.ts → 7/7 pass.
• tsc --build clean; prettier + eslint clean (0 errors).

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