fp-cuda: device-resident F₂ row reduction (RREF) on Hopper

[JoeyBF]

Stacked on #273 (the fp-cuda Hopper GEMM backend) — review/merge #273 first;
until it lands this PR's diff includes #273's commits. Rebases to blas3-only once
#273 merges.

Adds a fully device-resident F₂ (GF(2)) row reduction (reduced row echelon form)
on Hopper, dispatched from fp::Matrix::row_reduce for large p = 2 matrices,
and bit-exact vs the CPU M4RI oracle.

What's here

• CPU foundation: a blocked GEMM-based row-reduction design + prototype and
  benchmarks establishing the algorithm and the bit-exact oracle.
• Device port (one commit per phase): DeviceMatrix + on-device GEMM packing
  (Phase 3), panel_factor base kernel (Phase 4), device-resident forward
  row-echelon pass (Phase 5a), back-substitution to canonical RREF (Phase 5b),
  and dispatch of fp::Matrix::row_reduce to the device above a size threshold
  (Phase 7). The whole reduction stays on the GPU — uploaded once, downloaded
  once.
• 2^n scaling benchmarks + an effective binary-op throughput metric (Tbop/s).
• Optimization campaign (each pass its own commit): cooperative multi-CTA
  panel_factor / promote / block_reduce, wide adaptive forward panels,
  active-row compaction, blocked-TRSM back-substitution (now default) with a
  small-grid reduce, grid-cap tuning, and a low-barrier grid_sync. Flag-gated
  net-loss/wash experiments (deep recursion, blocked-TRSM promote, recursive
  panel) are kept as distinct commits documenting the exploration.
• Cleanup: retires the experiments + FP_CUDA_PROF instrumentation, drops
  the A/B kill-switches (commits to the proven defaults), keeps the tuning knobs,
  and re-validates the dispatch threshold at 8192.

Testing (H200)

• cargo test -p fp --features gpu --test cuda_dispatch — gpu_row_reduce_matches_cpu
  and gpu_dispatch_matches_cpu pass (bit-exact vs CPU).
• All fp-cuda demos (row_reduce_demo, forward_reduce_demo, panel_factor_demo,
  gemm_xor_into_demo, matmul_b1_dev_demo) print all-OK.
• Throughput: ~234 Tbop/s at 2^16, ~488 at 2^17 (half-rank square).

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