V2 quantizer: fix IO-boundary shared clusters left in float

[rascani]

Summary:
Shared-op clusters (e.g. cat, view, reshape) on the quantized IO boundary were silently left in float by the composable TOSA quantizer (_TOSAQuantizerV2), causing them to fall off the Ethos-U integer delegate onto CPU.

SharedQspecQuantizer propagates a qspec only from already-quantized neighbors. A cluster whose only quantized neighbors are a uint8 model input (intentionally skipped by _skip_shared_qspec_from_io to confine uint8 to the IO boundary) and/or an input-state placeholder with no output_qspec had no qspec to propagate, so it was rejected and remained in float.

The fix adds _is_quantized_io_boundary, which detects annotated placeholder/output nodes that signal the cluster is on the quantized data path even when their qspec is filtered. _get_shared_clique now returns a touches_quantized_io flag alongside the usual results. When _annotate_shared_cluster finds an empty adjacent_qspecs but a boundary-touching cluster, it initiates quantization from the global config input-activation qspec instead of rejecting. _TOSAQuantizerV2.set_global now also propagates to shared_qspec_quantizer.global_config so the fallback is wired automatically.

This restores the correctness fix from D107320847, which was abandoned because its other fix (parameter-operand weight misclassification) had already been resolved via the is_weight PARAMETER_TARGETS refactor.

This change was developed with assistance from Claude.

Differential Revision: D108662081

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell

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❌ 3 New Failures

As of commit d405304 with merge base e257a71 ():

NEW FAILURES - The following jobs have failed:

• Cadence Build & Test / hifi-build / hifi4 (gh)
  Input required and not supplied: aws-region
• Cadence Build & Test / vision-build / vision (gh)
  Input required and not supplied: aws-region
• pull / test-qnn-models-linux (dl3) / linux-job (gh)
  RuntimeError: Command docker exec -t 445986001e83e356516bf39a2f64cb38dca5e3016ea1250cb32cc4b073a647f4 /exec failed with exit code 92

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