fix(synthesis): namespace lead ids by kind to stop collisions (audit C-11)

Each stage (find_connections/gaps/improvements/propose_conjectures) numbered its
leads from seq=1, so connection L-0001, gap L-0001 and improvement L-0001 all
resolved to grounded/L-0001.md. The CLI concatenates them
(connections + gaps + improvements) and write_leads writes in order, so later
kinds silently overwrote earlier ones — survivors = max(per-kind count), not the
sum. Reproduced: 4 distinct leads -> 2 files.

_make_lead_id now takes the kind and emits L-C-/L-G-/L-I-/L-X- prefixes, keeping
same-seq ids distinct across stages. Regression tests cover the format and the
no-collision invariant.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/synthesis/test_grounded.py

@@ -76,7 +76,7 @@ def test_finalise_grounded_lead_succeeds_when_grounded(
     )
     assert lead is not None
     assert lead.kind == "connection"
-    assert lead.id == "L-0001"
+    assert lead.id == "L-C-0001"  # kind-prefixed id (audit C-11)
     assert lead.confidence >= 0.5
     assert lead.status == "unverified"
     assert lead.suggested_validation == ""