Unify default emitter around universal LogEvent.header

All event kinds now share one shape: a one-line header plus an optional
diff body for changes. LogEvent.header is a lazy property covering every
kind (change, aborted, created, migrated), and default_emit reduces to
logging the header plus routing diff_lines to the .diff child logger.
Custom emitters can tap the same blocks - header, diff_lines, Line,
format_diff - instead of reimplementing formatting per message type.

tests/test_logemit.py

@@ -290,3 +290,28 @@ async def test_aborted_transaction_includes_resolved_user(
     messages = [r.getMessage() for r in caplog.records if r.levelno == logging.WARNING]
     assert any(" by " in m and "Alice" in m for m in messages)
     assert any(" transaction aborted: boom" in m for m in messages)
+
+
+def test_event_header_covers_all_kinds():
+    created = LogEvent(kind="created", logger=transaction_logger, filename="x.db")
+    assert created.header == "Created x.db"
+
+    migrated = LogEvent(
+        kind="migrated",
+        logger=transaction_logger,
+        filename="x.db",
+        from_version=0,
+        to_version=1,
+        migrations=["migrate_v1 (rename)"],
+    )
+    assert migrated.header == "Migrated x.db v0 -> v1: migrate_v1 (rename)"
+
+    aborted = LogEvent(
+        kind="aborted",
+        logger=transaction_logger,
+        action="reset",
+        user="alice",
+        error=ValueError("boom"),
+    )
+    assert "transaction aborted: boom" in aborted.header
+    assert "alice" in aborted.header