feat: add stratified_undersample helper function

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/dataset_builder.py

@@ -459,3 +459,33 @@ def generate_dataset_vectorized(
     print(f"  최종 데이터셋: {n:,}개 (시그널={total_sig:,}, HOLD={total_hold:,})")
 
     return feat_final
+
+
+def stratified_undersample(
+    y: np.ndarray,
+    source: np.ndarray,
+    seed: int = 42,
+) -> np.ndarray:
+    """Signal 샘플 전수 유지 + HOLD negative만 양성 수 만큼 샘플링.
+
+    Args:
+        y: 라벨 배열 (0 or 1)
+        source: 소스 배열 ("signal" or "hold_negative")
+        seed: 랜덤 시드
+
+    Returns:
+        정렬된 인덱스 배열 (학습에 사용할 행 인덱스)
+    """
+    pos_idx = np.where(y == 1)[0]                                    # Signal Win
+    sig_neg_idx = np.where((y == 0) & (source == "signal"))[0]       # Signal Loss
+    hold_neg_idx = np.where(source == "hold_negative")[0]             # HOLD negative
+
+    # HOLD negative에서 양성 수 만큼만 샘플링
+    n_hold = min(len(hold_neg_idx), len(pos_idx))
+    rng = np.random.default_rng(seed)
+    if n_hold > 0:
+        hold_sampled = rng.choice(hold_neg_idx, size=n_hold, replace=False)
+    else:
+        hold_sampled = np.array([], dtype=np.intp)
+
+    return np.sort(np.concatenate([pos_idx, sig_neg_idx, hold_sampled]))

tests/test_dataset_builder.py

@@ -249,3 +249,20 @@ def test_signal_samples_preserved_after_sampling(signal_producing_df):
     signal_count = (result_with_hold["source"] == "signal").sum()
     assert signal_count == len(result_signal_only), \
         f"Signal 샘플 손실: 원본={len(result_signal_only)}, 유지={signal_count}"
+
+
+def test_stratified_undersample_preserves_signal():
+    """stratified_undersample은 signal 샘플을 전수 유지해야 한다."""
+    from src.dataset_builder import stratified_undersample
+
+    y      = np.array([1, 0, 0, 0, 0, 0, 0, 0, 1, 0])
+    source = np.array(["signal", "signal", "signal", "hold_negative",
+                        "hold_negative", "hold_negative", "hold_negative",
+                        "hold_negative", "signal", "signal"])
+
+    idx = stratified_undersample(y, source, seed=42)
+
+    # signal 인덱스: 0, 1, 2, 8, 9 → 전부 포함
+    signal_indices = np.where(source == "signal")[0]
+    for si in signal_indices:
+        assert si in idx, f"signal 인덱스 {si}가 누락됨"