Reduce duplicate InteractionC timeline events

The interaction record's creation date normally trails its start date
by milliseconds, so serializing both nearly doubled the timeline with
duplicate entries. Only emit the creation date when it diverges from
the start date by more than an hour, with explicit wording, since a
record created long after its event indicates backfill by sync,
restore or tampering.

Per-contact aggregate dates from ZCONTACTS repeat on every interaction
row of the same contact and carried that row's message text. Serialize
them with contact-centric data strings instead, so timeline
de-duplication collapses them into one first/last-seen event per
contact.

tests/ios_backup/test_interactionc.py

@@ -56,6 +56,37 @@ class TestInteractionCModule:
             "Bob Example (+14155550101) to local user" in events
         )
 
+        # The creation date is only serialized when it diverges from the
+        # start date; the SMS record was created 90 days after the event.
+        creation_events = [
+            entry
+            for entry in m.timeline
+            if entry["event"] == "interactions_creation_date"
+        ]
+        assert len(creation_events) == 1
+        assert creation_events[0]["timestamp"] == "2025-12-09 12:26:40.000000"
+        assert creation_events[0]["data"] == (
+            "Interaction record created 90 days after the event: "
+            "[com.apple.MobileSMS] INCOMING from "
+            "Bob Example (+14155550101) to local user"
+        )
+
+        # Per-contact aggregate dates use contact-centric data strings.
+        first_seen = [
+            entry
+            for entry in m.timeline
+            if entry["event"] == "first_incoming_sender_date"
+        ]
+        assert len(first_seen) == 1
+        assert first_seen[0]["timestamp"] == "2025-09-03 13:46:40.000000"
+        assert first_seen[0]["data"] == (
+            "First incoming interaction from Bob Example (+14155550101)"
+        )
+        assert (
+            "Last incoming interaction from Bob Example (+14155550101)"
+            in events
+        )
+
     def test_extraction_without_whatsapp_contacts(self):
         # Without the WhatsappContacts dependency the module still runs, and
         # unresolvable LIDs are shown as-is.