Add camera motion blur to WebP renders

[slimbuck]

Summary

Adds opt-in camera motion blur for .webp output. Enabled by passing --camera-end; the renderer averages N sub-frames with the camera linearly interpolated between the start pose (--camera, --look-at, --up) and the end pose (--camera-end, --look-at-end, --up-end).

New CLI flags (all under "IMAGE OUTPUT (.webp)"):

• --camera-end <x,y,z> — end camera position; presence enables motion blur.
• --look-at-end <x,y,z> — end target. Default: same as --look-at.
• --up-end <x,y,z> — end up vector. Default: same as --up.
• --shutter <0..1> — fraction of the start→end segment integrated, centered on the midpoint (standard shutter-angle convention: 1.0 = full motion, 0.5 = 180° shutter). Default: 1.
• --motion-samples <n> — sub-frames to accumulate. Default: 16.

Disabled by default — without --camera-end the existing single-sample path runs unchanged.

Approach

Motion blur is a thin orchestration layer in writeImage: no shader, rasterizer, or render-pass changes. For each sub-frame i ∈ [0, N) we lerp position + target, nlerp up, recompute DoF uniforms against the interpolated pose (so focus distance follows the look-at point), call the existing renderSplats(), and accumulate the returned RGBA buffer in a Float32Array. Final pixels are round(accum / N).

DoF (--f-stop etc.) composes naturally — each sub-frame applies its own per-splat covariance dilation, and the average is a physically-reasonable approximation of simultaneous defocus + motion blur. Equirect projection is also supported.

Files touched: src/lib/types.ts, src/lib/writers/write-image.ts, src/cli/index.ts, src/lib/write.ts, README.md.

Test plan

• npm run build clean
• npm run lint clean
• npm test — all 491 tests pass, including the 3 render-golden cases (tiny, mid, tiny-dof)
• Regression check: without --camera-end, output is byte-identical to current main (covered by goldens)
• Idempotence: --camera-end equal to --camera with --motion-samples 16 produces a bit-identical WebP to the single-sample render
• --shutter 1 vs --shutter 0.5 on a dolly produces visibly shorter streaks at 0.5
• --camera-end + --f-stop 2.8 composes both effects in one frame
• --projection equirect --camera-end ... produces a panoramic frame with motion blur

Example invocation:

splat-transform input.ply view.webp \
    --camera 0,1,-3 --look-at 0,0,0 \
    --camera-end 1,1,-3 \
    --motion-samples 16 --shutter 1

[Copilot]

Pull request overview

Adds opt-in camera motion blur for .webp rendering by interpolating camera poses across multiple sub-frame renders and averaging the result.

Changes:

• Extends render options and CLI flags for end camera pose, shutter fraction, and motion sample count.
• Updates writeImage to build per-sample cameras, recompute DoF per pose, accumulate RGBA output, and encode the averaged result.
• Documents the new flags and adds a README example.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/lib/writers/write-image.ts	Implements motion-blur camera interpolation and frame accumulation for WebP output.
src/lib/write.ts	Passes new render motion-blur options into the image writer.
src/lib/types.ts	Adds public option fields for motion-blur rendering controls.
src/cli/index.ts	Adds CLI parsing, validation, and help text for motion-blur flags.
README.md	Documents new WebP motion-blur options and example usage.

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