feat(dqn): enable Branching DQN with 45 factored actions (5×3×3)

Restore 45-action factored space via Branching DQN (Tavakoli 2018),
outputting 11 Q-values (5+3+3) instead of 45. This was reduced to 5
exposure-only actions during debugging and was never intended as permanent.

- Enable use_branching: true by default in DQNConfig and DQNHyperparameters
- Add branching paths to select_action_with_confidence and select_action_inference
- Update agent.rs select_action_factored for branching-aware selection
- Expand CountBonus to per-branch tracking with bonuses_branched()
- Add order_type + urgency distribution tracking in monitoring
- Add DQN_ORDER_ACTIONS=3, DQN_URGENCY_ACTIONS=3, DQN_TOTAL_ACTIONS=45 to CUDA header
- Fix 7 pre-existing clippy doc_markdown errors in regime_conditional.rs
- Fix pre-existing cognitive_complexity in replay_buffer_type.rs (extract helpers)
- Fix flaky GPU test OOM under parallel execution (CPU fallback + test VRAM safety)
- Delete unused flash_attention submodules (block_sparse, causal_masking, etc.)
- Add GPU hot-path guard scripts and ensemble/hyperopt adapter improvements

Tests: ml-dqn 416/0, ml 905/0, clippy 0 errors on both crates

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/pre-commit-hook.sh

@@ -57,10 +57,23 @@ if [ -n "$STAGED_FILES" ]; then
     fi
 fi
 
+# GPU hot-path leak detection
+echo "🔎 Checking for GPU→CPU leaks in hot paths..."
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+if [ -x "$SCRIPT_DIR/gpu-hotpath-guard.sh" ]; then
+    if ! "$SCRIPT_DIR/gpu-hotpath-guard.sh" --staged; then
+        echo "⛔ GPU hot-path leak detected — fix or suppress with // gpu-ok: <reason>"
+        exit 1
+    fi
+    echo "   No GPU→CPU leaks in hot paths"
+    echo ""
+fi
+
 echo "✅ All pre-commit checks passed!"
 echo ""
 echo "Summary:"
 echo "  - Code quality checks: ✅"
+echo "  - GPU hot-path guard: ✅"
 echo ""
 
 exit 0