Add transparent and matte background modes

[JeffLtz]

What

Adds two output modes that expose the matte as real transparency, so a consumer can composite the isolated subject over their own surface (a WebGL scene, another canvas, the DOM) instead of one of the built-in backgrounds:

• transparent — RGBA = (subject.rgb · a, a): the subject on a transparent background (premultiplied), matte as the canvas alpha channel.
• matte — RGBA = (a, a, a, a): the raw alpha as a white silhouette; a debug view and a reusable mask.

new EffectsPipeline(stream, { background: 'transparent' })
// or, to preview onto a page-owned canvas while `main` stays untouched:
pipeline.attachPreview(canvas)
await pipeline.setPreview({ background: 'transparent' })

Why

Today the SDK only emits an opaque composite (subject over blur/image/color), and the output MediaStream can't carry alpha. There was no way to get just the subject out to composite it yourself. This came out of putting a webcam subject in front of a MilkDrop-style WebGL visualizer — the cleanest path was to have longpipe hand back the subject on transparency and stack it, rather than chroma-key a synthetic background.

How

No context changes were needed — the WebGL context is already alpha:true/premultiplied and the WebGPU canvas is configured alphaMode:'premultiplied'; the stock compositors just hardcode alpha = 1.0. This mirrors composite_solid across both backends:

• new GLSL / WGSL shaders (+ f16 WGSL variants) and per-backend ops
• CompositorTransparent / CompositorMatte effect wrappers
• CompositeSpec transparent / matte modes wired through buildCompositor, sameSpec, and renderer.translateBackgroundFor
• 'transparent' / 'matte' keywords in BackgroundInput / Background + normalizeBackground
• backend.presenters interface + WebGL/WebGPU registration
• README backgrounds section

Because the network + alpha are shared across main/preview and a preview effect already forces inference, main:'none' + preview:'transparent' renders the isolated subject to a page canvas efficiently (the MediaStream output remains opaque, as documented).

Tests

tests/effects/composite_alpha.test.ts:

• WebGL: pixel-exact premultiplied output — transparent → (255,0,0,255) / (0,0,0,0), matte → (255,255,255,255) / (0,0,0,0).
• WebGPU: compile-and-submit smoke (guarded on adapter availability).

npm run typecheck and npm run build pass.

🤖 Generated with Claude Code

[JeffLtz]

Hi Sam! In case it's not obvious this whole PR was generated by one of my agents so critical feedback is very very welcome.

You can check out what I made here:

https://butterchurn-rust.vercel.app/

And if you check "Isolate subject" you'll see how I incorporated Longpipe.