Web shell: don't self-clamp maxStorageBuffersPerShaderStage to the WebGPU default

[octopusburrow]

The WebGPU shell's wantedLimits hardcoded maxStorageBuffersPerShaderStage at 8 —
exactly the WebGPU default — so Math.min(wanted, adapter) could never grant more even
on adapters offering 16+. The Mobile rendering method (Godot's default renderer for web
exports) has a scene shader that binds 10 storage buffers in
the vertex stage, so every lit 3D scene failed CreatePipelineLayout with a
GPUValidationError naming this limit, and rendered black. (Unshaded scenes stay under
the limit, which is why the samples that render are the unshaded ones.)

Request the adapter's own maximum for this limit instead, matching the policy the
engine.js device path already uses. Measured on Windows/Dawn (adapter max 16): the
pipeline-layout errors disappear and all Mobile scene pipelines create cleanly.

Honest scope note: this does not yet produce lit pixels. With the limit fixed and zero
remaining validation errors, scenes with lights and materials still render black on the
Mobile renderer; only unshaded scenes produce output (verified against an unshaded
control through the same harness, under both directional and pure-ambient lighting).
The limit clamp was the first blocker; a second, silent one remains, and we're
investigating. This fix stands on its own: the validation errors are real, and any
lit-path fix needs it in place first.