fix(tests): gpu_backtest_validation action constants — Long100 is 72, not 4

Two `gpu_backtest_validation` tests were failing with bit-identical
deterministic values for 2+ months: `test_always_long_on_downtrend`
(expected negative PnL, got +0.00023627281) and
`test_multiple_windows_produce_results` (uptrend < downtrend instead
of > ).

Root cause: SP21 Phase 8.5 (2026-05-12) wired the factored 4-3-3-3
action decoder into the eval (`backtest_state_gather` + env_step),
but the test's hardcoded `constant_action_model(4, ...)` integer
literal wasn't migrated. Pre-Phase-8.5 the eval used a flat
4-action enum where `4` reportedly meant Long100; the factored
decoder now interprets `4` as:

  decode_direction_4b(4, b1=3, b2=3, b3=3) = 4 / 27 = 0 = DIR_SHORT
  decode_magnitude_4b(4, b1=3, b2=3, b3=3) = (4/9) % 3 = 0 = MAG_QUARTER

So the test was running Short-Quarter (-0.25 position) on the trend
fixtures. On random-walk synthetic prices with drift ±0.001 vs σ=0.01
noise per bar, the 24-step eval window has S/N ≈ 0.49 — specific
seeds can produce net-against-drift trajectories, making the actual
short-quarter PnL small but deterministic, with sign flipped relative
to test intent.

Fix: change `constant_action_model(4, ...)` → `constant_action_model(72, ...)`
in the 2 failing tests. Action 72 = dir=LONG (2) * 27 + mag=FULL (2)
* 9 + 0 + 0 — the actual "Long100" under 4-3-3-3 factoring. Both
tests now pass; no regressions on the 4 previously-passing tests.

Verification
────────────
- gpu_backtest_validation pre-fix: 4 passed, 2 failed
- gpu_backtest_validation post-fix: 6 passed, 0 failed

Out of scope for this commit (follow-up audit needed)
─────────────────────────────────────────────────────
Three other tests in the same file have the same stale `4` constant
with misleading "Always Long100" comments, but currently pass
incidentally:

- `test_always_long_on_uptrend` (line 218): asserts `total_pnl > 0`.
  Currently passes BY ACCIDENT — action=4 (Short-Quarter) on seed-42's
  net-down 24-bar trajectory produces +PnL, satisfying the assertion
  for the wrong reason. Fixing to action=72 alone would break this
  test (true Long100 on seed-42's net-down trajectory is negative);
  the test needs BOTH the action fix AND a seed/window change so the
  "uptrend" trajectory actually trends up over the eval window
  (e.g., 250-bar window or drift=0.01).
- `test_extended_metrics_populated` (line 465) and
  `test_active_model_records_trades` (line 524): assertions are
  direction-agnostic (VaR/CVaR/Calmar/Omega NaN-check + CVaR≤VaR;
  total_trades > 0 + win_rate range), so they pass legitimately
  under whatever-direction action=4 produces. The "Long100" comments
  are misleading but the tests are correctly covering their stated
  behavior.

A separate audit-and-fix pass should address all three at once:
either correct the action constants + adjust seed/window to ensure
each test's named trajectory direction is statistically reliable,
or introduce a named constant (e.g., LONG100_ACTION) and helper to
prevent the same drift recurring.

Refs
────
- SP21 T2.2 Phase 8.5 commit 5694eb4df: "wire factored-action branch
  sizes into closure-based eval (atomic)"
- crates/ml/src/cuda_pipeline/trade_physics.cuh: `decode_direction_4b`,
  `decode_magnitude_4b` — canonical factored action decoders
- crates/ml/src/cuda_pipeline/gpu_backtest_evaluator.rs:189:
  `DqnBacktestConfig::from_network_dims` — sets branch_sizes (4,3,3,3)

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

crates/ml/tests/gpu_backtest_validation.rs

@@ -284,7 +284,14 @@ mod gpu_tests {
         // env_step decoder. Match production eval-baseline.rs.
         evaluator.set_branch_sizes(&DqnBacktestConfig::from_network_dims((256, 256, 128, 128)));
 
-        let model = constant_action_model(4, &stream); // Always Long100
+        // Action 72 = Long×Full×order=0×urgency=0 under 4-3-3-3 factored
+        // decoding (action = dir*27 + mag*9 + order*3 + urgency = 2*27 +
+        // 2*9 + 0 + 0). Pre-Phase-8.5 the eval used a flat 4-action enum
+        // where `4` reportedly meant Long100, but SP21 Phase 8.5
+        // (2026-05-12) wired the factored decoder where `4` now decodes
+        // to Short-Quarter (dir=0, mag=0). The "Always Long100" comment
+        // was correct intent but the constant wasn't migrated.
+        let model = constant_action_model(72, &stream); // Always Long100 (factored)
         let metrics = evaluator
             .evaluate(&model, 24)
             .expect("evaluation should succeed");
@@ -405,7 +412,9 @@ mod gpu_tests {
         // env_step decoder. Match production eval-baseline.rs.
         evaluator.set_branch_sizes(&DqnBacktestConfig::from_network_dims((256, 256, 128, 128)));
 
-        let model = constant_action_model(4, &stream); // Always Long100
+        // Action 72 = Long×Full×order=0×urgency=0 under 4-3-3-3 factored
+        // decoding (see test_always_long_on_downtrend for full derivation).
+        let model = constant_action_model(72, &stream); // Always Long100 (factored)
         let metrics = evaluator
             .evaluate(&model, 24)
             .expect("evaluation should succeed");