fix(migrations): Preserve event cutover journal order

Keep the applied conversation-events schema entry at its main-branch ledger position, then journal the retained context, visible-message, and usage migrations in deployment order.

Freeze the rebased data migration implementations against the current event schema and update bootstrap coverage for mixed journals.

Author: dcramer

packages/junior/migrations/0009_conversation_history_to_sql.ts

@@ -73,10 +73,24 @@
       "breakpoints": true
     },
     {
-      "idx": 9,
+      "idx": 10,
       "version": "7",
-      "when": 1784137322425,
-      "tag": "0009_conversation_usage",
+      "when": 1784137322426,
+      "tag": "0010_conversation_context_checkpoints",
+      "breakpoints": true
+    },
+    {
+      "idx": 11,
+      "version": "7",
+      "when": 1784137322427,
+      "tag": "0011_conversation_visible_message_events",
+      "breakpoints": true
+    },
+    {
+      "idx": 12,
+      "version": "7",
+      "when": 1784137322428,
+      "tag": "0012_conversation_usage",
       "breakpoints": true
     }
   ]