Add vector assignment execution tests

[sudonatalie]

Adds CTS execution tests for vector assignment statements, including component access, constant and dynamic indexing, compound assignment, whole-vector assignment, and evaluation order across address spaces and memory views.

Issue: https://crbug.com/544734265

---

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 9878c04):

+webgpu:shader,execution,statement,assignment,vector:vector_component:* - 15 cases, 684 subcases (~46/case)
+webgpu:shader,execution,statement,assignment,vector:vector_constant_index:* - 15 cases, 342 subcases (~23/case)
+webgpu:shader,execution,statement,assignment,vector:vector_dynamic_index:* - 30 cases, 684 subcases (~23/case)
+webgpu:shader,execution,statement,assignment,vector:vector_element_compound:* - 24 cases, 576 subcases (~24/case)
+webgpu:shader,execution,statement,assignment,vector:vector_full_assignment:* - 45 cases, 342 subcases (~8/case)
+webgpu:shader,execution,statement,assignment,vector:indexed_assignment_eval_order:* - 1 cases, 1 subcases (~1/case)
+webgpu:shader,execution,statement,assignment,vector:indexed_compound_assignment_eval_order:* - 1 cases, 1 subcases (~1/case)
-TOTAL: 285384 cases, 2347172 subcases
+TOTAL: 285515 cases, 2349802 subcases