feat(sp20): scaffold 7 behavioral test stubs (Phase 0)

All 7 tests marked #[ignore] until corresponding Phase 6 task implements.
Tests gate L40S deployment per spec §4.6.

- sp20_pure_trend (Phase 6.1)
- sp20_pure_noise (Phase 6.2)
- sp20_confidence_correlation (Phase 6.3)
- sp20_asymmetry_no_game (Phase 6.4)
- sp20_n_step_distribution (Phase 6.5)
- sp20_per_regime_wr (Phase 6.6)
- sp20_regime_transition (Phase 6.7)

crates/ml/tests/sp19_20_behavioral_tests.rs

@@ -0,0 +1,49 @@
+//! SP19+20 Behavioral Test Suite — gates L40S deployment per spec §4.6.
+//!
+//! Reuses SP15 Phase 2A LobBar harness. Each test constructs a synthetic
+//! market, runs the model for N bars, and asserts behavioral invariants.
+//!
+//! Run all: `SQLX_OFFLINE=true CUDA_COMPUTE_CAP=86 cargo test -p ml --test sp19_20_behavioral_tests`
+//! Run one: `SQLX_OFFLINE=true CUDA_COMPUTE_CAP=86 cargo test -p ml --test sp19_20_behavioral_tests -- sp20_pure_trend`
+
+#[test]
+#[ignore = "Phase 0 stub — implementation in Phase 6.1"]
+fn sp20_pure_trend() {
+    unimplemented!("Phase 6.1");
+}
+
+#[test]
+#[ignore = "Phase 0 stub — implementation in Phase 6.2"]
+fn sp20_pure_noise() {
+    unimplemented!("Phase 6.2");
+}
+
+#[test]
+#[ignore = "Phase 0 stub — implementation in Phase 6.3"]
+fn sp20_confidence_correlation() {
+    unimplemented!("Phase 6.3");
+}
+
+#[test]
+#[ignore = "Phase 0 stub — implementation in Phase 6.4"]
+fn sp20_asymmetry_no_game() {
+    unimplemented!("Phase 6.4");
+}
+
+#[test]
+#[ignore = "Phase 0 stub — implementation in Phase 6.5"]
+fn sp20_n_step_distribution() {
+    unimplemented!("Phase 6.5");
+}
+
+#[test]
+#[ignore = "Phase 0 stub — implementation in Phase 6.6"]
+fn sp20_per_regime_wr() {
+    unimplemented!("Phase 6.6");
+}
+
+#[test]
+#[ignore = "Phase 0 stub — implementation in Phase 6.7"]
+fn sp20_regime_transition() {
+    unimplemented!("Phase 6.7");
+}