test(4branch): 3 unit tests — action encoding, position mapping, flat shortcut

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/ml/src/trainers/dqn/smoke_tests/reward_v8.rs

@@ -118,4 +118,56 @@ mod tests {
         assert!(norm_mean.abs() < 1e-10, "Mean should be 0, got {norm_mean}");
         assert!((norm_var - 1.0).abs() < 0.01, "Variance should be 1, got {norm_var}");
     }
+
+    // ── 4-Branch Architecture Tests ─────────────────────────────────────
+
+    #[test]
+    fn test_4branch_action_encoding() {
+        // All 81 actions encode/decode correctly
+        for dir in 0..3_usize {
+            for mag in 0..3_usize {
+                for order in 0..3_usize {
+                    for urgency in 0..3_usize {
+                        let action = dir * 27 + mag * 9 + order * 3 + urgency;
+                        assert_eq!(action / 27, dir);
+                        assert_eq!((action % 27) / 9, mag);
+                        assert_eq!((action % 9) / 3, order);
+                        assert_eq!(action % 3, urgency);
+                        assert!(action < 81);
+                    }
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn test_4branch_position_mapping() {
+        let max_pos = 2.0_f64;
+        // direction: 0=Short(-1), 1=Flat(0), 2=Long(+1)
+        // magnitude: 0=Small(0.25), 1=Medium(0.5), 2=Full(1.0)
+        // Short × Full = -2.0
+        assert!(((- 1.0_f64) * 1.0 * max_pos - (-2.0)).abs() < 1e-10);
+        // Flat × anything = 0
+        assert!((0.0_f64 * 0.25 * max_pos).abs() < 1e-10);
+        assert!((0.0_f64 * 1.0 * max_pos).abs() < 1e-10);
+        // Long × Quarter = +0.5
+        assert!((1.0_f64 * 0.25 * max_pos - 0.5).abs() < 1e-10);
+        // Long × Half = +1.0
+        assert!((1.0_f64 * 0.5 * max_pos - 1.0).abs() < 1e-10);
+        // Long × Full = +2.0
+        assert!((1.0_f64 * 1.0 * max_pos - 2.0).abs() < 1e-10);
+        // Short × Small = -0.5
+        assert!(((- 1.0_f64) * 0.25 * max_pos - (-0.5)).abs() < 1e-10);
+    }
+
+    #[test]
+    fn test_flat_shortcut() {
+        // Flat direction (idx=1) × any magnitude = 0 position
+        for mag in 0..3_usize {
+            let dir_fraction = 0.0_f64; // Flat
+            let mag_scale = match mag { 0 => 0.25, 1 => 0.5, _ => 1.0 };
+            assert!((dir_fraction * mag_scale).abs() < 1e-10,
+                "Flat × mag={mag} must be zero");
+        }
+    }
 }