fix(metal): preserve texture array copy shape

[besmpl]

Summary

• represent WebGPU 1D/2D array textures with Metal arrayLength and physical depth 1
• copy array layers as Metal slices while preserving true 3D Z/depth semantics
• validate complete Queue.WriteTexture source ranges before any Metal effect, including compressed block geometry
• use dimension-correct Metal row/image strides and checked copy arithmetic

Why

The previous Metal descriptor stored DepthOrArrayLayers in depth for every texture dimension, and copy paths used origin Z as both a texture slice and a region Z coordinate. That is correct for true 3D textures but not for 1D/2D arrays. It can create the wrong physical texture shape and copy only or address the wrong array layer. This also blocked a portable standard texture-array page in downstream renderers.

Scope

This is an independent Metal correctness fix. It contains no multi-draw, prepared-command, ICB, material-page, or Hearth API surface.

Verification

• go test -count=1 ./hal/metal
• focused physical copy/upload matrix repeated 20 times
• CGO_ENABLED=0 go test ./...
• git diff --check HEAD^..HEAD

Physical tests cover 1D arrays, 2D arrays, true 3D, both mixed 3D↔2D-array copy directions, private and shared uploads, non-zero origins/offsets, padded rows/images, and truncated final layers. Full race/vet remain blocked by pre-existing Objective-C/goffi unsafe/checkptr diagnostics outside this diff.