perf(hex-poly-fast): remove Karatsuba traversal allocation

[kim-em]

Closes #9744

Summary

• replace per-diagonal List.range folds with allocation-free index loops while preserving the raw schoolbook specification
• carry an offset through unbalanced Karatsuba block traversal so each block is copied once instead of copying every remaining suffix
• retain proofs through ofCoeffs, benchmark both kernels against their former implementations, and synchronize the performance-report registry

Verification

• lake build HexPolyFast.Karatsuba HexPolyFast.Conformance hexpolyfast_bench hexpolyfast_emit_fixtures
• fresh conformance fixture output matches conformance-fixtures/HexPolyFast/polyfast.jsonl
• oracle fallback checked all 120 emitted cases (local python-flint unavailable)
• lake exe hexpolyfast_bench verify passes all 57 local targets; the seven FLINT-backed fixed targets fail only because python-flint is unavailable locally
• Phase-4 headline/registration guard passes for all four new targets
• focused warm-cache comparisons at n=1024: diagonal list 24.213 ms vs loop 7.457 ms; at 64:1 n=256: suffix-copy 114.626 ms vs offset 28.791 ms; paired hashes agree

[kim-em]

Fresh Claude Opus second opinion completed while CI was running.

Accepted and addressed:

• made the retained old raw comparators generic over the same CommRing interface as production
• normalized all four raw benchmark outputs through ofCoeffs before hashing
• synchronized the performance-report registry and added the required explicit cost-model derivations

Independently verified: the source/proof chain is sound, Raw.ofCoeffs_schoolbook and Raw.ofCoeffs_blocks remain intact, and the named list/suffix allocations are removed.

Not expanded in this PR: a single mutable output buffer for addShift, a block-count scaling campaign, and narrowing the diagonal loop bounds. Those are broader/pre-existing optimization questions; #9744 specifically requests an allocation-free diagonal loop and offset-based block traversal, and no theorem or conformance obligation was weakened.