feat: add auditable Trikona Shodhana

(cherry picked from commit 9531d1dcc70d1c3e2b70c9627ac077d47d484b92)

scripts/ashtakavarga.py

@@ -586,3 +586,37 @@ def calc_sodhita_av(bav_results: Dict, planets: Dict, asc_sign_idx: int) -> Dict
         'reduction_summary': total_reduction,
         'validation_note': 'Sodhita AV ≤ 原始BAV（逐宫位）',
     }
+
+
+TRIKONA_SIGN_GROUPS = (
+    (0, 4, 8),   # Aries, Leo, Sagittarius
+    (1, 5, 9),   # Taurus, Virgo, Capricorn
+    (2, 6, 10),  # Gemini, Libra, Aquarius
+    (3, 7, 11),  # Cancer, Scorpio, Pisces
+)
+
+
+def calc_trikona_shodhana(sav_scores: List[int]) -> Dict:
+    """Apply the classical Trikona Shodhana reduction to a 12-sign SAV row.
+
+    The minimum value in each elemental trine is removed from all three signs;
+    the original row remains in the result for auditability.
+    """
+    if len(sav_scores) != 12:
+        raise ValueError('SAV scores must contain exactly 12 signs')
+    original = [int(value) for value in sav_scores]
+    reduced = original[:]
+    groups = []
+    for indexes in TRIKONA_SIGN_GROUPS:
+        baseline = min(original[index] for index in indexes)
+        for index in indexes:
+            reduced[index] -= baseline
+        groups.append({'sign_indexes': list(indexes), 'baseline_removed': baseline})
+    return {
+        'method': 'Trikona Shodhana',
+        'original_scores': original,
+        'reduced_scores': reduced,
+        'trikona_groups': groups,
+        'status': 'executed',
+        'claim_boundary': 'Standalone classical reduction; Ekadhipatya Shodhana remains a separate layer.',
+    }