viewer: expose snapshot render channels

[thomas-yanxin]

What does this PR do?

Adds a backward-compatible captureChannels() API to the viewer snapshot pipeline. It returns same-camera color, albedo, view-normal, and linear camera-range depth captures for visual evidence and diagnostic consumers.

• Reuses the existing scene MRT; the scene geometry is still rendered once.
• Creates albedo/depth conversion pipelines and targets lazily, so existing capture() calls pay no extra resource cost.
• Keeps display color as WebP and encodes the three data views losslessly as PNG.
• Leaves the existing capture() API and output behavior unchanged.

How to test

1. Run bun check.
2. Run bun run check-types.
3. Run bun run test.
4. Start bun dev and confirm the editor and IFC routes load.

Additional browser smoke coverage rendered a lit box and floor through the WebGL fallback, then verified all four captures share dimensions, have the declared MIME types, and contain non-empty, visually distinct color/albedo/normal/depth data.

Screenshots / screen recording

No UI changes. The browser smoke test visually verified aligned color, unlit albedo, packed view-normal, and linear depth outputs.

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (run bun check to verify)
• I've documented the public channel semantics inline
• This PR targets the main branch

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Note

Medium Risk
Touches GPU render targets, MRT readback, and WebGPU/WebGL pixel paths, but existing capture() callers are unchanged and new resources are lazy.

Overview
Adds captureChannels() on the snapshot pipeline so consumers can get color, albedo, view-normal, and linear depth from the same camera in one call, alongside unchanged capture() (still WebP display color only).

Channel metadata is exported (SNAPSHOT_CHANNELS, per-channel MIME: WebP for color, PNG for data buffers). Pixel readback, crop/resize, and WebGPU vs WebGL2 row handling are centralized in readCapture, reused by both APIs. Albedo and depth use lazy extra render pipelines/targets; capture() alone does not allocate them until captureChannels() runs.

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