test(sp15-p1.0): scaffold sp15_phase1_oracle_tests.rs for Phase 1

Empty mod gpu placeholder; Phase 1 tasks 1.1-1.7 will append per-task
oracle tests. No tests yet — file just must compile.

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

crates/ml/tests/sp15_phase1_oracle_tests.rs

@@ -0,0 +1,8 @@
+// crates/ml/tests/sp15_phase1_oracle_tests.rs
+//! SP15 Phase 1 oracle tests — honest numbers (sharpe, cost-net, DD,
+//! baselines, dd_pct trunk grounding, test slice consumption).
+
+#[cfg(test)]
+mod gpu {
+    // Per-task tests append here; see plan §Phase 1 task 1.X.
+}