Forward packed QKV and normalization kind for APB

[Supernova-45]

Summary

• Forward the optional packed Q/K/V projection parameters from the public
  cuequivariance_torch.attention_pair_bias API to the ops backend.
• Expose the backend's optional norm_kind contract while preserving
  layer_norm as the public default.
• Omit the default normalization keyword at dispatch so a new public package
  remains compatible with an older ops wheel; explicit RMSNorm still requires
  backend support.

This is public-API plumbing only. It does not change kernels, routing policy,
or behavior for existing callers that do not use the new options.

Motivation

RFD4's production geometry uses a packed QKV projection and RMSNorm. Without
these arguments in the public wrapper, the integration can only reach the
optimized APB backend by calling the ops package directly.

Validation

• Complete repository pre-commit suite: worker job 3561046 (clean).
• Focused public-wrapper forwarding/default test: 1 passed.
• Mixed-version compatibility is exercised by the RFD4 contract test: the
  default LayerNorm path works when the installed ops backend lacks both new
  keywords.
• Exact public-wrapper/RFD4 route qualification on locked H200: job 3561137
  at public head 6fdca1d, RFD4 head a5f4cf5, and backend head 462e9ad2.
  The actual RFD4 class compiled to cuequivariance.apb_flash_forward; at
  B=8/N=384 it measured 1.361x over compiled-native RMSNorm with output
  max-absolute error 4.88e-4 and all-gradient max-absolute error 2.74e-7.

Backend implementation and qualification: kernelcatcher MR !686.
Integration consumer: baker-laboratory/RFD4-Proteina-dev#224.

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[Supernova-45]

@greptileai review

@coderabbitai review

[Supernova-45]

Final review disposition:

• Exact-head CodeRabbit CLI review completed with zero findings.
• Greptile produced no inline findings; its 4/5 summary refers to VERSION/release-section state that is not in this four-file PR diff, so I did not expand this API-plumbing PR with unrelated release-version edits.
• All nine GitHub checks pass. Worker job 3561046 ran the complete all-files pre-commit suite and focused wrapper contract.
• Locked-H200 job 3561137 exercised the final public wrapper through the actual RFD4 class and selected cuequivariance.apb_flash_forward (1.361x for the APB block at B=8/N=384, with output/all-gradient checks in the PR description).

Ready for human review.