fix(dasha): judge antardasha sandhi from the unclipped start

Upstream 3c2dad95, rewritten for this tree. A birth-clipped antardasha start was reported as a transition. The first mahadasha start stays, because BUG-1268 already stores the real pre-birth boundary there. BUG-1382.

Co-Authored-By: Grok (agent loop)
Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_dasha_sandhi_birth_clip.py

@@ -0,0 +1,60 @@
+"""A clipped birth balance is not an actual Dasha transition.
+
+Upstream ``tests/test_dasha_sandhi_birth_clip.py`` (3c2dad95) skips the first
+mahadasha start whenever ``is_balance`` is set. This tree does not: BUG-1268
+already stores the real pre-birth mahadasha boundary in ``start``.
+
+| 处 | 原值 | 新值 | 原因 |
+| --- | --- | --- | --- |
+| 第一大运 ``start`` | ``2000-01-01``，并跳过 ``is_balance`` 起点 | ``1998-01-01``，不跳过 | 我方第一大运起点是出生前的真实交接 |
+| 断言 | 跳过起点后窗口为空 | 起点超出容限，窗口仍为空 | 单独证明被裁剪的小运起点不算交接 |
+"""
+from types import SimpleNamespace
+
+from scripts.jyotish_engine import _calc_dasha_sandhi, cmd_dasha
+
+
+def test_birth_clip_does_not_create_a_transition():
+    data = {'timeline': [{'lord': 'Saturn', 'is_balance': True,
+        'start': '1998-01-01', 'end': '2010-01-01',
+        'antardasha_timeline': [{'lord': 'Mercury', 'start': '2000-01-01', 'end': '2002-01-01',
+            'unclipped_start_datetime': '1999-01-01T12:30:00'}]}]}
+    assert _calc_dasha_sandhi(data, '2000-01-01')['sandhi_windows'] == []
+
+
+def test_actual_unclipped_start_is_still_found():
+    data = {'timeline': [{'lord': 'Saturn', 'is_balance': True,
+        'start': '1998-01-01', 'end': '2010-01-01',
+        'antardasha_timeline': [{'lord': 'Mercury', 'start': '2000-01-01', 'end': '2002-01-01',
+            'unclipped_start_datetime': '1999-12-20T12:30:00'}]}]}
+    windows = _calc_dasha_sandhi(data, '2000-01-01', 30)['sandhi_windows']
+    assert len(windows) == 1
+    assert windows[0]['date'] == '1999-12-20'
+    assert windows[0]['days_from_reference'] == -12
+
+
+def test_real_boundaries_are_inclusive_at_orb_limit():
+    data = {'timeline': [{'lord': 'Sun', 'start': '2000-01-01', 'end': '2000-02-01'}]}
+    windows = _calc_dasha_sandhi(data, '2000-01-02', 1)['sandhi_windows']
+    assert len(windows) == 1 and windows[0]['date'] == '2000-01-01'
+
+
+def test_prebirth_mahadasha_start_is_not_discarded():
+    data = {'timeline': [{'lord': 'Saturn', 'is_balance': True,
+        'start': '1999-12-01', 'end': '2010-01-01',
+        'antardasha_timeline': []}]}
+    windows = _calc_dasha_sandhi(data, '2000-01-01', 90)['sandhi_windows']
+    assert [row['date'] for row in windows] == ['1999-12-01']
+
+
+def test_thirty_day_chart_does_not_list_the_birth_day_as_an_antardasha_join():
+    packet = cmd_dasha(SimpleNamespace(
+        moon_lon=20 / 3, nakshatra=None, pada=None,
+        birthdate='2000-01-01', year=2000, month=1, day=1, hour=12, minute=0, second=0,
+        lat=None, lon=None, tz=0, today='2000-01-31', dasha_year_days=365.25))
+    windows = _calc_dasha_sandhi(packet, '2000-01-31', 30)['sandhi_windows']
+    clipped = [
+        row for row in windows
+        if row['level'] == 'antardasha' and row['boundary'] == 'start' and row['date'] == '2000-01-01'
+    ]
+    assert clipped == []