fix(synthesis): enforce non-empty provenance + fix gap lead + clean index docstring

- Lead.__post_init__: raises ValueError on empty provenance list
- find_gaps: no-chunk path now carries topic-marker Provenance instead of []
- _update_index: docstring corrected (rebuilds, not append-only)
- test_model: test_lead_empty_provenance_raises covers the new invariant

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

tests/synthesis/test_model.py

@@ -78,6 +78,19 @@ def test_lead_missing_provenance_field_raises() -> None:
         )
 
 
+def test_lead_empty_provenance_raises() -> None:
+    """Constructing Lead with empty provenance list raises ValueError."""
+    with pytest.raises(ValueError, match="at least one Provenance"):
+        Lead(
+            id="L-0004",
+            kind="connection",
+            title="x",
+            body="y",
+            provenance=[],
+            confidence=0.5,
+        )
+
+
 def test_lead_id_format_helper() -> None:
     """The internal _make_lead_id helper produces zero-padded L-XXXX strings."""
     from codex.synthesis import _make_lead_id