fix(sp21): T2.2 Phase 8.5 — wire factored-action branch sizes into closure-based eval (atomic)

v7 smoke (train-fv4s8, commit 23b89a90e) eval pod hit
CUDA_ERROR_ILLEGAL_ADDRESS at fold 0:

  Error: DQN fold 0 GPU evaluation failed: GpuBacktestEvaluator::evaluate
    failed for fold 0: Model error: eval_done_event synchronize:
    DriverError(CUDA_ERROR_ILLEGAL_ADDRESS, "an illegal memory access was encountered")

The {:#} anyhow chain fix from Phase 8.3+9 made the failure mode visible.
Diagnosis from code (no second smoke needed): the closure-based
evaluate() path never sets b0_size..b3_size, leaving them at the
default 0. env_step kernel's decode_*_4b helpers do action/(b1*b2*b3)
→ divide-by-zero → garbage decoded indices → out-of-bounds memory
read → CUDA_ERROR_ILLEGAL_ADDRESS at next event-sync.

The production `evaluate_dqn_graphed` path sets b-sizes via
`ensure_action_select_ready` (which also lazy-allocates intent buffers
the closure path doesn't need). The closure-based `evaluate()` path
used by eval-baseline never calls it.

Fix:

  1. Add pub fn `GpuBacktestEvaluator::set_branch_sizes(&mut self,
     dqn_cfg: &DqnBacktestConfig)` — sets b0..b3_size only, no
     buffer allocation.

  2. Add defensive guard in `evaluate()` that bails with
     `MLError::ConfigError` if any b-size is zero. Future regressions
     produce a clear error instead of an opaque CUDA illegal-address.

  3. Wire `set_branch_sizes(&dqn_cfg)` call in
     `evaluate_dqn_fold_gpu` between `DqnBacktestConfig::from_network_dims`
     and the closure-based `evaluator.evaluate(...)`.

Pearls honoured:
  - feedback_no_hiding: zero-b-size now surfaces as ConfigError
    rather than CUDA illegal-address downstream
  - feedback_no_partial_refactor: closure-path was a partial wire-up
    from pre-factored-action days; set_branch_sizes brings it into
    parity with the CUBLAS production path for action decoding
  - pearl_no_deferrals_for_complementary_fixes: v7's chain-exposing
    fix surfaced this; lands immediately not after another smoke

Verification:
  cargo check -p ml --example evaluate_baseline --features cuda  # clean

Note on PPO/supervised paths:
  Their evaluate() calls also lack set_branch_sizes and will now
  trip the defensive guard. Those paths haven't actually run eval
  since STATE_DIM grew past 54 — the silent failure mode had been
  masking it. Future Phase will either wire their action conventions
  (PPO: 5-exposure; supervised: signal thresholds) or delete the
  dead paths per feedback_no_partial_refactor.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/ml/examples/evaluate_baseline.rs

@@ -832,6 +832,16 @@ fn evaluate_dqn_fold_gpu(
 
         let dqn_cfg = ml::cuda_pipeline::gpu_backtest_evaluator::DqnBacktestConfig::from_network_dims(network_dims);
 
+        // Phase 8.5 (2026-05-12) — wire factored-action branch sizes into the
+        // evaluator's env_step kernel. Without this, env_step's
+        // decode_direction_4b / decode_magnitude_4b / decode_order_4b /
+        // decode_urgency_4b helpers divide by zero (b1*b2*b3 = 0), and the
+        // GPU raises CUDA_ERROR_ILLEGAL_ADDRESS asynchronously at the next
+        // event-sync. The production `evaluate_dqn_graphed` path sets these
+        // via `ensure_action_select_ready`; this closure-path equivalent
+        // calls the public `set_branch_sizes` setter.
+        evaluator.set_branch_sizes(&dqn_cfg);
+
         {
             // Standalone eval: closure-based forward pass.
             // Training eval uses FusedTrainingCtx as QValueProvider for determinism.