Make ShapeColorBuffer not pixel/voxel-specific

[mkeeter]

It's convenient to construct a single ShapeColorBuffers object and use it for both pixel and voxel evaluation; we'll just make sure the config region at the start is large enough for both.

[Copilot]

🟢 Approval recommended

The refactor is cohesive and the shader/host layout invariants are guarded by dedicated layout/size tests in both pixel and voxel paths.

Pull request overview

This PR refactors ShapeColorBuffers so a single buffer instance can be reused across both pixel and voxel color evaluation paths by standardizing the size of the “config header” region that precedes the tape data.

Changes:

• Move ShapeColorBuffers construction from pixel/voxel effects contexts to a new top-level Gpu::color_buffers API.
• Remove the ShapeColorBuffers<C> generic parameter and standardize the config-header size used when packing config + tape data.
• Update the pixel color config padding (Rust + WGSL) and add tests to ensure pixel/voxel config header sizes match.

File summaries

File	Description
fidget-wgpu/src/voxel/mod.rs	Update tests to construct ShapeColorBuffers via Gpu.
fidget-wgpu/src/voxel/effects/mod.rs	Remove context-local color_buffers, switch APIs to &ShapeColorBuffers, and add config-size test.
fidget-wgpu/src/pixel/mod.rs	Update tests to construct ShapeColorBuffers via Gpu.
fidget-wgpu/src/pixel/effects/shaders/color.wgsl	Adjust config padding in WGSL to keep tape-data offset consistent with shared buffer layout.
fidget-wgpu/src/pixel/effects/mod.rs	Remove context-local color_buffers, switch APIs to &ShapeColorBuffers, update ColorConfig padding, and add config-size/layout tests.
fidget-wgpu/src/lib.rs	Add Gpu::color_buffers, make ShapeColorBuffers non-generic, and centralize expected config-header sizing.
CHANGELOG.md	Document the API move for ShapeColorBuffers construction.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 1
• Review effort level: Lite

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