Per spec §2.1 + §5.4 point 1 and Task 10 of the plan.
Adds binding `cfc_step_per_branch_fwd_gpu` in cfc/step.rs that wraps
the fused per-branch kernel with the spec-mandated launch config:
grid=(B, N_HORIZONS=5, 1), block=(MAX_BUCKET_DIM=28, 1, 1) uniform
predicate, cooperative staging of x_local + h_old_local (2 × HIDDEN_DIM
floats). Also adds `cfc_step_per_branch_bwd_gpu` binding for Task 11.
CfcTrunk loads two new cubins (cfc_step_per_branch + heads_block_diagonal)
and caches three function handles. `heads_w_skip_compact_d` (from Task 9)
remains allocated as Phase-2-prepared metadata; full heads consumption
deferred to a follow-up task (Phase 2 heads dispatch needs GRN
integration, which is out of Task 10 scope per
`feedback_no_partial_refactor`).
In `dispatch_train_step`, the CfC dispatch branches on TrainingPhase:
- Phase1Warmup: existing `cfc_step_batched` (unchanged)
- Phase2Routed: `cfc_step_per_branch_fwd` consuming bucket-routed
`tau_all_d` + `bucket_channel_offset_d` + `bucket_dim_k_d`
GRN heads dispatch stays unchanged for both phases per the Task 10
scope adjustment (Option B in the task brief). The `heads_w_skip` storage
remains 640 floats; the compact 128-float `heads_w_skip_compact_d` is
metadata-ready but not yet consumed.
Workspace cargo check clean (ml-alpha lib + trainer compile; only
pre-existing cupti/sp15/gpu_per_integration test errors remain).
ml-alpha lib tests: 33 passed.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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