Final cleanup: lazy logfmt build, docstring and example fixes

- transaction.py only builds the logfmt chain when logging is enabled
- callbacks.py module docstring documents the logemit special case
- docs logemit example uses the settable header instead of a user lookup
  that would break when logfmt resolution is registered

docs/database.md

@@ -269,9 +269,9 @@ def resolve_user_key(value: str) -> str | None:
 def emit(ev: LogEvent):
     if ev.kind != "change":
         return default_emit(ev)  # delegate, no chaining needed
-    actor = ev.current.get("users", {}).get(ev.user, {}).get("name", ev.user)
-    line = Line().user(actor, width=20)(" ").action(ev.action)
-    ev.logger.log(ev.level, f"{line}\n" + "\n".join(ev.diff_lines))
+    # Restyle the header; default_emit keeps routing the diff body.
+    ev.header = str(Line().user(ev.user or "-", width=20)(" ").action(ev.action))
+    return True
 ```
 
 #### Terminal Formatting Helpers

kanta/callbacks.py

@@ -7,6 +7,10 @@ default value.
 Log formatters are a special case: they are called per value being rendered
 and receive the value plus an optional ``path`` string.  They return
 ``str | None``; ``None`` means "fall through to the next formatter".
+
+Log emitters (``logemit``) are another special case: plain callables that
+receive a :class:`kanta.logging.LogEvent` and are dispatched by
+:func:`kanta.logging.emit_event`.
 """
 
 from __future__ import annotations

kanta/transaction.py

@@ -90,8 +90,8 @@ def transaction(
             previous = impl.statedict
             record = impl.queue_change(action, new_dict, user=user, mtime=mtime)
             if record is not None:
-                logfmt = _build_logfmt(impl, previous, new_dict)
                 if log is not False:
+                    logfmt = _build_logfmt(impl, previous, new_dict)
                     logger = (
                         log if isinstance(log, logging.Logger) else transaction_logger
                     )