Adds CTS test coverage for the snorm10-10-10-2 vertex format.

[wangra-google]

• Adds snorm10-10-10-2 to kVertexFormatInfo in capability_info.ts (with an explicit cast until @webgpu/types is updated).
• Adds signed 10-10-10-2 bit-packing (makeRgb10a2Signed) and normalization (normalizeRgb10a2Signed) test helpers.
• Adds packed format test data in correctness.spec.ts to verify conversion from raw vertex data to vec4<f32> in the vertex shader.

Issue: gpuweb/gpuweb#6292

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Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located.
• Test descriptions are accurate and complete.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Tests avoid over-parameterization (see case count report).

When landing this PR, be sure to make any necessary issue status updates.

[github-actions]

Results for build job (at 2b8d1a8):

-webgpu:api,operation,vertex_state,correctness:vertex_format_to_shader_format_conversion:* - 164 cases, 1476 subcases (~9/case)
-webgpu:api,operation,vertex_state,correctness:setVertexBuffer_offset_and_attribute_offset:* - 41 cases, 1444 subcases (~35/case)
-webgpu:api,operation,vertex_state,correctness:non_zero_array_stride_and_attribute_offset:* - 41 cases, 777 subcases (~19/case)
+webgpu:api,operation,vertex_state,correctness:vertex_format_to_shader_format_conversion:* - 168 cases, 1512 subcases (~9/case)
+webgpu:api,operation,vertex_state,correctness:setVertexBuffer_offset_and_attribute_offset:* - 42 cases, 1472 subcases (~35/case)
+webgpu:api,operation,vertex_state,correctness:non_zero_array_stride_and_attribute_offset:* - 42 cases, 792 subcases (~19/case)
-webgpu:api,operation,vertex_state,correctness:vertex_buffer_used_multiple_times_overlapped:* - 41 cases, 369 subcases (~9/case)
-webgpu:api,operation,vertex_state,correctness:vertex_buffer_used_multiple_times_interleaved:* - 41 cases, 369 subcases (~9/case)
-webgpu:api,operation,vertex_state,correctness:max_buffers_and_attribs:* - 41 cases, 41 subcases (~1/case)
-webgpu:api,operation,vertex_state,correctness:array_stride_zero:* - 41 cases, 686 subcases (~17/case)
+webgpu:api,operation,vertex_state,correctness:vertex_buffer_used_multiple_times_overlapped:* - 42 cases, 378 subcases (~9/case)
+webgpu:api,operation,vertex_state,correctness:vertex_buffer_used_multiple_times_interleaved:* - 42 cases, 378 subcases (~9/case)
+webgpu:api,operation,vertex_state,correctness:max_buffers_and_attribs:* - 42 cases, 42 subcases (~1/case)
+webgpu:api,operation,vertex_state,correctness:array_stride_zero:* - 42 cases, 700 subcases (~17/case)
-webgpu:api,operation,vertex_state,correctness:overlapping_attributes:* - 41 cases, 41 subcases (~1/case)
+webgpu:api,operation,vertex_state,correctness:overlapping_attributes:* - 42 cases, 42 subcases (~1/case)
-webgpu:api,validation,render_pipeline,vertex_state:vertex_shader_type_matches_attribute_format:* - 41 cases, 492 subcases (~12/case)
-webgpu:api,validation,render_pipeline,vertex_state:vertex_attribute_offset_alignment:* - 588 cases, 10584 subcases (~18/case)
 [snip - full report in action logs]
-TOTAL: 285339 cases, 2346439 subcases
+TOTAL: 285364 cases, 2347068 subcases