Cortex-M: add opt-in explicit layout lowering

[rascani]

Thread an experimental explicit-layout mode through the shared Edge compile configuration, AOT compiler, quantizer, tester, and pass manager while retaining legacy dim-order export as the default.

Explicit mode disables dim-order operators, converts Conv1d through the common transform, runs the common channels-last pipeline, lowers anchors to fixed _nhwc schemas, and lowers surviving boundaries to cortex_m transpose. Padding is retargeted explicitly, output dequantization remains a layout barrier, and unequal-shape add or multiply stays outside the region until separately admitted.

Package the host test harness and assert expected Cortex-M targets before planned-memory comparisons for M33 and M55.

Nine Cortex-M55 FVP cases, common-transform suites, and lintrunner.

Authored with Codex.

[rascani]

Stack from ghstack (oldest at bottom):

• Cortex-M: keep NHWC channel broadcasts quantized #21968
• -> Cortex-M: add opt-in explicit layout lowering #21967
• Cortex-M: add explicit-NHWC channel-broadcast ops #21966
• Cortex-M: add explicit-NHWC runtime operators #21965
• Cortex-M: share layout-aware CMSIS-NN kernels #21964
• Channels last: add the common layout pipeline #21963
• Channels last: optimize structural layout regions #21962
• Channels last: preserve metadata across op replacement #21961

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21967

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❌ 3 New Failures, 1 Unrelated Failure

As of commit 17b573e with merge base 1b2838b ():

NEW FAILURES - The following jobs have failed:

• pull / test-qnn-buck-build-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 7a37cd01fb18d220c2abcd86cee9f083c1b209c957c0fad56132b3115902049b /exec failed with exit code 3
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 1a2d4aed60b8f86f216408fb46de6bc341d0d16794d1cf5d14cf6231c18ccd50 /exec failed with exit code 3
• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Apple / build-demo-ios / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 65

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