Lazy clock reads; demo: auto-advancing +1h clock, module-level setup

docs/database.md

@@ -168,6 +168,8 @@ when they have a default value.
 - `@kanta.clock` registers a callback `() -> datetime` that replaces the
   default UTC clock.  Its value is used for all record timestamps (`ts`, and
   `m` when `mtime` is `True`) and for snapshot timestamps.
+- The clock is only read when a timestamp is actually produced; no-op
+  transactions and skipped snapshot checks do not read it.
 - Register before `open()` so that bootstrap and migration records use the
   custom clock as well.  This is mainly useful for tests and reproducible
   demos.