Demo: deterministic clock, # section banners, .kantadb extension, raw record dump

docs/database.md

@@ -163,6 +163,15 @@ when they have a default value.
 - Multiple handlers are supported and invoked in registration order.  A failing
   handler is logged and does not prevent subsequent handlers from running.
 
+#### Clock
+
+- `@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.
+- Register before `open()` so that bootstrap and migration records use the
+  custom clock as well.  This is mainly useful for tests and reproducible
+  demos.
+
 #### Transaction Log Formatting
 
 - Logfmt callbacks prettify identifiers in the change log and are registered with