[ET-VK][runtime][1/3] Prepare linear shaders for 2D dispatch

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [ET-VK][ops] Add batch support to q8ta convolutions #22254
• [ET-VK][runtime][3/3] Enforce device-safe dispatch geometry #22225
• [ET-VK][runtime][2/3] Add typed dispatch size abstractions #22224
• -> [ET-VK][runtime][1/3] Prepare linear shaders for 2D dispatch #22223
• [ET-VK][ops] Move group norm reduction lanes to Z #22222

Add dispatch.glslh with linear_idx_from_gid(), which flattens two-dimensional global invocation IDs. Migrate shaders whose global X coordinate represents a linear logical index. Existing one-dimensional dispatch geometry remains unchanged, making this a behavior-neutral preparation for centralized dispatch wrapping.

Authored with Codex.

Differential Revision: D117706349

[pytorch-bot]

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❌ 2 New Failures, 1 Unrelated Failure

As of commit ed8a598 with merge base a5f15b5 ():

NEW FAILURES - The following jobs have failed:

• pull / test-llama-runner-linux (fp32, xnnpack+quantize_kv, linux.arm64.2xlarge, executorch-ubuntu-22.04-... / linux-job (gh)
  RuntimeError: Command docker exec -t e85e1bf887aee86e3d89062067e523f5168adc2ae9760ce5d8dc8a1604871217 /exec failed with exit code 1
• pull / test-qnn-models-linux (mv3) / linux-job (gh)
  Error: Input required and not supplied: path

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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