Cortex-M: add explicit-NHWC channel-broadcast ops

[rascani]

Add fixed-schema quantized_add_nhwc and quantized_mul_nhwc entry points for channel broadcasts inside explicit NHWC regions.

The legacy schemas continue interpreting dimension one as channels, while the new wrappers select dimension three through shared CMSIS-NN implementations. Register the Python references, runtime schemas, selective-build entries, and layout-parity coverage.

Direct add and multiply FVP execution on Cortex-M55 and Cortex-M33, selective-runner cross-builds, 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]

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❌ 3 New Failures, 1 Unclassified Failure

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• pull / test-qnn-buck-build-linux / linux-job (gh)
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• pull / unittest-buck / linux / linux-job (gh)
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• pull / unittest-buck / macos / macos-job (gh)
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