PGOLF-619: add public CUDA BF16 train-visible execution posture

[AtlantisPleb]

Parent: #188, #183

Why this exists:

• the repo has BF16 policy and accounting posture, but the public CUDA runtime still does not honestly expose broad BF16 train-visible tensor, gradient, and optimizer execution
• the single-H100 bring-up keeps refusing this blocker explicitly

Scope:

• widen the public CUDA/runtime surface needed for BF16-forward or FP32-master Parameter Golf training
• keep storage/compute/accumulator/master-weight posture machine-readable and explicit
• add targeted tests or evidence for the bounded BF16 train path that actually lands

Exit criteria:

• the public Rust-owned Parameter Golf lane has a real BF16-or-master-weight CUDA execution path instead of only policy receipts
• the cuda_bf16_train_precision_contract blocker is retired or materially narrowed with explicit evidence

[AtlantisPleb]

Landed on main in bc1a74e.

What changed:

• psionic-backend-cuda now owns dense BF16 buffer residency plus bounded row-major BF16xBF16->F32 matmul execution through the public CUDA runtime.
• the canonical Parameter Golf CUDA coverage truth now narrows the BF16 blocker from cuda_bf16_train_precision_contract to cuda_bf16_train_graph_and_optimizer_surface.
• the dependent Parameter Golf reports were regenerated so the single-H100 bring-up, submission evidence, final PR bundle, local-clone dry run, and record-track contract all carry the narrowed BF16 blocker honestly.
• docs/PARAMETER_GOLF_CUDA_TRAINING_COVERAGE.md and docs/ROADMAP_PARAMETERGOLF.md now mark PGOLF-619 done and leave PGOLF-623 open for the remaining BF16 train-graph/optimizer closure.

Validation:

• cargo test -p psionic-backend-cuda cuda_submission_executes_ -- --nocapture
• cargo test -p psionic-train parameter_golf_cuda_training_capability_report_tracks_required_families -- --nocapture
• cargo test -p psionic-train parameter_golf_submission_run_evidence_report_matches_committed_truth -- --nocapture --test-threads=1
• cargo test -p psionic-train parameter_golf_record_track_contract_report_matches_committed_truth -- --nocapture --test-threads=1
• cargo test -p psionic-train single_h100_bringup -- --nocapture
• cargo test -p psionic-train committed_local_clone_dry_run_report_stays_green -- --nocapture --test-threads=1
• cargo test -p psionic-train parameter_golf_distributed_8xh100_lane_measures_ddp_posture -- --nocapture
• cargo test -p psionic-eval parameter_golf_distributed -- --nocapture
• scripts/check-parameter-golf-single-h100-bringup.sh
• scripts/check-parameter-golf-acceptance.sh