feat(attention): add CP backward drift validation

[inaniloquentee]

Scope

PR8 fixes the strict backward schedule for CP prefill and chunked-prefill.

Contract

Area	Enforced behavior
Gradient order	Canonical batch, query, and global-KV traversal
Reduction	FP32 accumulation with fixed logical merge order
Outputs	Covers dQ, dK, and dV, including CP ownership
Split-KV	Disabled in strict backward mode

Validation

Check	Result
Strict backward suite	57 passed
Integrated 8xH100 backward gate	Passed: dQ/dK/dV max abs 0.0 across 2/4/8 ranks

Boundary

This PR owns backward arithmetic order and gradient coverage. PR285 and #294 record the production H100 evidence.

Commit: 9e5c400

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