Add equirectangular projection for WebP output and refactor compute shaders into chunks

[slimbuck]

Summary

Adds a native 360°×180° equirectangular projection mode to the WebP image output (--projection equirect) so users can export a panorama of a Gaussian-splat scene from a single camera position, and reorganizes the GPU rasterizer's eight compute shaders out of one ~1600-LOC file into per-shader source files plus 14 small WGSL chunks under src/lib/gpu/shaders/, using PlayCanvas's cincludes/cdefines preprocessor (see engine/src/platform/graphics/shader.js) so chunk files are pure WGSL and projection/SH-band variants flip via #ifdef PROJECTION_EQUIRECT / #ifdef SH_BAND_{1,2,3}.

The equirect path is implemented inside the existing tile-binning GPU rasterizer rather than as a cube-face stitch: the project shader swaps the perspective screen mapping and EWA Jacobian for spherical (atan2/asin) equivalents, the CPU depth sort uses radial distance, the tile-bin emit-pairs shader wraps the X tile range across the ±π longitude seam (modular tile indices), and the rasterize-binned shader wraps per-pixel dx into [-W/2, W/2] so seam-straddling splats pull their footprint from the nearer copy. A POLE_EPS clamp on rxz keeps the latitude Jacobian bounded near zenith/nadir. The structure follows the PlayCanvas engine's own fisheye-projection compute path in compute-gsplat-common.js as a precedent.

The chunk refactor consolidates the JS↔WGSL uniform layout into a single co-located uniforms.ts (struct chunk + matching UniformFormat[]), moves scalar tunables from render/config.ts (TILE_SIZE, SIGMA_CUTOFF, JACOBIAN_LIMIT_FACTOR, etc.) into a shared WGSL const chunk so the shader files contain no ${...} template interpolation, and extracts the projection screen-mapping, Jacobian, tile-AABB, SH-band-evaluation, quaternion-to-rotation, and 3D-covariance blocks into named chunks. gpu-splat-rasterizer.ts shrinks from ~1600 to ~800 LOC (orchestrator class only).

A small follow-up replaces magic literals in gpu-dilation.ts (0u/1u/2u/0x9E3779B9u) with named BLOCK_EMPTY/BLOCK_SOLID/BLOCK_MIXED/FIBONACCI_HASH WGSL consts for readability in the extract and compact shaders.

Test plan

• npm run lint && npm run build && npm test — full suite (490 tests) passes.
• Render-golden tests byte-identical: node --import tsx --test test/render-golden.test.mjs produces byte-exact matches for both committed fixtures (tiny.webp 320×240 and mid.webp 640×360), confirming the pinhole path is unchanged.
• Equirect end-to-end: splat-transform input.ply --projection equirect --camera 0,1,0 --look-at 0,1,1 out.webp (default 2048×1024), then open in a panorama viewer (e.g. Pannellum) and verify the horizon sits at mid-image, the --look-at direction maps to image-x = W/2, and the left/right edges of the image are continuous (no visible seam).
• Equirect input validation: --fov with --projection equirect errors out; non-2:1 --resolution errors out; default resolution is 2048×1024 when neither width nor height is supplied.
• GPU dilation byte-identical: splat-transform input.ply --voxel-params 0.5,0.1 --voxel-external-fill 1.6 --seed-pos 100,100,100 out.voxel.json produces a byte-identical .voxel.json + .voxel.bin pair to a pre-PR run on main.

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