Add a WebGPU backend alongside WebGL2

[DanKane2029]

Adds a WebGPU backend alongside the existing WebGL2 one — not replacing it. The same ECS world, camera, editor, and shaders' semantics; two interchangeable GPU drivers behind one interface. The Renderer menu in the nav bar switches between them, and "Automatic" prefers WebGPU with a silent fallback to WebGL2.

Targeted at main directly (see CLAUDE.md's note on this repo's stacked-PR history).

Two commits, reviewable in order

1. 2fbee78 — Lift GPU handles off the assets, define the RenderBackend seam. No behavior change; this is the enabling refactor and the bulk of the risk.

GPU handles used to live on the asset objects: VertexBuffer held a WebGLBuffer and a VAO, Shader a WebGLShader, Texture a WebGLTexture, each with created/loaded flags. That married every mesh to one context — Renderer.ts's own comments cited it as why a second context was rejected — and left no field a WebGPU handle could occupy. Handles now live in a backend-owned cache keyed by the asset, so assets are pure CPU-side data.

Also here, because the interface has to fit both APIs honestly:

• Attribute locations pinned in attributeLocations.ts and bound before link, instead of queried per-program. A mesh's VAO no longer depends on any shader program, and WGSL's numeric @location slots can agree with GLSL's names.
• pick() and readTargetPixels() are async on both backends. WebGPU can only map memory back asynchronously. GizmoController.onMouseDown awaits the pick and guards a stale answer with a request id plus a button-still-down check.
• Picker.ts folds into WebGLBackend (it needed the private framebuffer). App.picker/App.renderer → App.pick/App.backend.

2. 75628da — The WebGPU backend. Full parity: lit + unlit shading, textures, selection outline, GPU picking, and the Render panel's offscreen final render.

The four things that bite

• Clip space. mat4.perspective builds an OpenGL projection (z ∈ [-1,1]); WebGPU wants [0,1]. Handed over unchanged it silently clips away the near half of the view volume — reads like a broken near plane, not a coordinate convention. CLIP_SPACE_CORRECTION fixes it, and a test pins near→0 / far→1.
• Uniforms are raw bytes. UniformLayout.ts must match the WGSL structs exactly, including a vec3 occupying 16 bytes and a mat3x3 occupying 48, not 36. A wrong offset renders a subtly wrong scene rather than throwing, so this is tested directly.
• Cull mode is baked into a pipeline. WebGL flips gl.cullFace(FRONT) mid-frame for the outline; WebGPU needs a second pipeline bound partway through the same pass.
• A canvas's context type is permanent. Switching backends remounts the Scene panel's <canvas> (keyed on the backend choice) rather than reconfiguring it.

One change that reaches outside the backend

Every mesh now carries the same three attributes (standardVertexLayout), with zeroed UVs where a mesh has none. WebGL tolerated a missing attribute by feeding the shader (0,0,0,1); WebGPU rejects a pipeline that doesn't supply every location its shader declares. This makes explicit what WebGL was already substituting, and avoids compiling a shader variant per layout. It grows OBJ-loaded vertex buffers by 2 floats/vertex.

Shaders are hand-written WGSL twins (src/Shaders/wgsl/) rather than transpiled — a transpiler is a heavy build dependency whose output nobody could read, in a repo that exists to be read. A test pins the parts that can be checked without a GPU (MAX_LIGHTS across all four declarations, attribute locations).

Verification

• tsc --noEmit clean (strict on)
• 166/166 tests pass, 15 new across UniformLayout.test.ts and Backends.test.ts
• production webpack build clean; WGSL confirmed inlined into the bundle
• lint: no new problems (the one pre-existing Vector.test.ts error is untouched)

Not verified in a browser — this environment has no GPU or headless browser, so no frame has actually been rendered through either backend on this branch. Before merging, please run npm run start:dev and check, for each backend: scene draws lit and textured; orbit; click-select; the outline reads as a rim not a solid halo; gizmo drags in all three modes; the Render panel matches the viewport minus gizmo/camera icons; resize the panel then re-pick a corner object (catches id-target resize bugs); close and reopen the Scene panel; switch backends and repeat.