feat: add receive message reconciliation helper

docs/source-discovery/2026-07-10-receive-message-reconciliation-precheck.md

@@ -136,7 +136,7 @@ Reason:
 
 ## Next implementation node
 
-Recommended next business node: **R2 contact/group search quality hardening**.
+Recommended next business node after R12: continue R13/R14 business smoke and release-candidate closure unless new real evidence changes priority.
 
 Reason:
 
@@ -149,3 +149,28 @@ Future receive-specific node after R2/R3:
 - Add a local-only reconciliation helper that compares PacketReader and MsgLib messages by sanitized keys and outputs only match counts, source refs, and mismatch categories.
 - Do not implement pagination until reconciliation proves which source should drive ordering.
 - Do not implement file download until R3 proves message-to-cache/download mapping.
+
+
+## R12 implementation result
+
+R12 added a local-only reconciliation helper:
+
+- `internal/isphere/message_reconciliation.go`
+- `internal/isphere/message_reconciliation_test.go`
+- `scripts/verify-receive-source-reconciliation.ps1`
+
+Fixture verification:
+
+```json
+{"ok":true,"verification":"receive_source_reconciliation","use_fixture":true,"strong_matches":1,"medium_matches":1,"unmatched_left":1,"unmatched_right":1,"auto_merge_recommended":false,"exact_fixture_auto_merge_recommended":true,"raw_message_bodies_returned":false,"raw_rows_returned":false,"file_paths_returned":false,"default_receive_routing_changed":false}
+```
+
+The helper is pure comparison logic: it performs no DB reads, log reads, send, download, hook, injection, or writes.
+
+Current match behavior:
+
+- strong match: exact non-empty `message_id` on both sides;
+- medium match: same `conversation_id`, same `sender_id`, and timestamp within 3000 ms;
+- auto merge is recommended only for complete strong-match coverage with no unmatched or medium-only records.
+
+Decision remains unchanged: default `isphere_receive_messages` routing stays PacketReader/log-backed. `msglib_readonly` remains explicit only.