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.

docs/database.md

@@ -234,10 +234,16 @@ def resolve_user_key(value: str) -> str | None:
   `"migrated"`, `"aborted"`), the preferred `logger` and `level`, and all
   relevant state: `action`, `user`, `extra`, `error` (for aborted
   transactions), `diff`, `previous`/`current` state dicts, the built `logfmt`
-  chain, and version info for migration events.  The default `"aborted"`
-  rendering is `<action>[ by <user>] transaction aborted: <error>` with the
-  action and user colored and the user resolved via `logfmt`.  Pretty
-  `header` and `diff_lines` are lazy properties, built only if accessed.
+  chain, and version info for migration events.
+- The built-in formatting is assembled from standard blocks that custom
+  emitters can reuse as-is or replace piecemeal:
+  - `event.header` — a lazy property producing the default one-line header
+    for any kind: `<action>[ <extra>][ by <user>]` for changes,
+    `<action>[ by <user>] transaction aborted: <error>` for aborts, and the
+    plain `Created`/`Migrated` summaries.
+  - `event.diff_lines` — a lazy property producing the pretty diff body for
+    change events (built only if accessed).
+  - `default_emit` itself is just `header` plus the `diff_lines` routing.
 - `@kanta.logemit` registers a callback receiving the event.  The callback
   decides what is logged and where: it may log one or more messages on
   `event.logger`, log somewhere else, or nothing at all.  A falsy return