fix(nakshatra): keep the first mahadasha's antardashas on the full clock

Upstream 8643bc9a. The nakshatra breakdown was restarting the nine antardashas from the birth remainder, so mid-Ashwini reported Ketu instead of Rahu. Reuse cmd_dasha and clip to birth. BUG-1380.

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

tests/test_nakshatra_native_dasha_clock.py

@@ -0,0 +1,17 @@
+"""The interpretation helper must consume the real, unclipped MD/AD clock."""
+from scripts.nakshatra_dasha import calc_nakshatra_dasha_breakdown
+
+
+def test_first_remaining_md_is_not_repartitioned_into_new_ads():
+    # Mid-Ashwini: half of Ketu's seven years elapsed. The active AD is Rahu,
+    # not Ketu; the helper must not restart the first parent's child clock.
+    result = calc_nakshatra_dasha_breakdown(
+        20 / 3, {'Ketu': 1., 'Rahu': 181.},
+        '2000-01-01T12:34:56', 0.)
+    assert result['mahadasha']['lord'] == 'Ketu'
+    assert result['antardasha']['lord'] == 'Rahu'
+    assert result['antardasha']['start_datetime'] == '2000-01-01T12:34:56'
+    assert result['reference_datetime'] == '2000-01-01T12:34:56'
+    assert result['antardasha']['progress'] > 0
+    # Missing natal positions are never silently fabricated as 0 degrees.
+    assert {row['period'] for row in result['pada_windows']} <= {'AD Ketu', 'AD Rahu'}