Add actor_child_id to completion_events (D-008)

Per IMPLEMENTATION-ORDER.md 1.5: a nullable actor_child_id column
records which panel profile was tapped; admin events stay NULL.

- schema.py: completion_events table body extracted to
  COMPLETION_EVENTS_TABLE_SQL; the new column carries a FK to
  children(id) and the actor-pair coherence is extended to a second
  CHECK: 'user' events require actor_user_id and forbid
  actor_child_id, 'panel' events require actor_child_id and forbid
  actor_user_id.
- migrations.py: migration 6 rebuilds legacy-shaped completion_events
  via the canonical procedure (CHECKs cannot be added to an existing
  table), backfilling each legacy panel row to its own child_id
  (pre-D-008 panel completions were always the tapped child) via
  INSERT..SELECT — no UPDATE or DELETE path exists, so the append-only
  guarantee is untouched; ids preserved; PRAGMA foreign_key_check
  verified in-transaction.
- dao_instances.py: append() takes actor_child_id, validates both
  actor shapes (panel requires it, user forbids it) and the tapped
  child's existence under the connection lock; the record and column
  list grow accordingly.
- tests: coherence CHECK matrix (5 bad shapes), unknown
  actor_child_id FK, panel/user round-trips at DDL and DAO layers,
  migration-6 backfill test on a frozen v2-shape seed; the
  completion_events leak guard gains justified INSERT exemptions for
  the migration layer (UPDATE/DELETE stay forbidden everywhere).

tests/test_dao_matrix.py

@@ -372,14 +372,16 @@ def test_matrix_completion_events_append_read_and_no_mutation(
     async def _body(w: World):
         # insert: append a completion then a reversal, then re-complete
         await w.events.append(
-            w.instance.id, w.child.id, "completed", "panel", _stamp(), True
+            w.instance.id, w.child.id, "completed", "panel", _stamp(), True,
+            actor_child_id=w.child.id,
         )
         await w.events.append(
             w.instance.id, w.child.id, "uncompleted", "user", _stamp(), None,
             actor_user_id="user-1",
         )
         await w.events.append(
-            w.instance.id, w.child.id, "completed", "panel", _stamp(), False
+            w.instance.id, w.child.id, "completed", "panel", _stamp(), False,
+            actor_child_id=w.child.id,
         )
         # read: full ordered history + latest
         history = await w.events.list_by_instance(w.instance.id)