feat(webgpu): add instanced rect 2D rendering and raise texture limit

[jimwei]

Adds a compact instanced-rectangle 2D path to the WebGPU backend, plus a requestDevice texture-limit fix.

What changed

1. src/canvas.mbt - new runtime API rt.pushRectInst2d(rects, colors, palette, pipelineCode), a rectInst render command, two storage buffers (rect vec4<f32> + palette index), a dedicated bind group, pipelines for pipeline code 12/13, and a single pass.draw(6, rectCount) submission.
2. src/shader_sources.mbt - new instance-rect WGSL shader (index 10) that expands the 6 triangle corners in the vertex shader and samples the palette.
3. src/canvas.mbt - request maxTextureDimension2D from the adapter when calling requestDevice, so large glyph atlases (for example 12,288 px) are not rejected by the default device limit.

Motivation

The spreadsheet PoC renders >100k visible cells. The previous path expanded every rectangle into 6 vertices in JS and uploaded that vertex buffer every frame. The new path uploads 20 bytes per instance (rect + palette index) and submits all rectangles in one draw call.

Benchmarks (Chrome headless + WebGPU, 118,604-cell grid):

Scenario	pushColor2d baseline	pushRectInst2d
Large grid	~69 ms/frame, 1.72 M cells/s	~31.9 ms/frame, 3.72 M cells/s
Text pressure, 100 frames	925.9 ms	368.5 ms

Compatibility

The consuming app checks for rt.pushRectInst2d and falls back to the existing pushColor2d vertex-expansion path, so unpatched versions of the runtime remain supported.

Verification

The patched package is consumed and verified in the spreadmb PoC (verify_selene.py, verify_selene_unicode_edge.py, verify_cjk_selene.py). A standalone moon check --target js in selene-webgpu currently hits unrelated dependency parse errors in moonbitlang/x@0.4.45 under the latest MoonBit toolchain, so the runtime validation was done through the consuming project.