Add an input to the sacn-in node so the observed universe can be changed
at runtime. On an incoming msg.universe (integer 1-63999) the node stops
listening on the current universe (removeUniverse) and starts listening on
the new one (addUniverse), drops the stale cache and updates the node
status. Invalid values warn and are ignored; an unchanged value is a no-op.

Document the new input in docs.html and rebuild dist.

Introduce an output trigger that is independent of the merge mode:
- changes: emit only when values change
- always: emit on every received sACN packet, even without a change
- interval: emit on change and, as a keepalive, re-emit the full universe
  when no change arrives within a configurable interval (default 1000 ms)

Existing nodes without the field keep their previous behaviour
(passthrough=always, htp/ltp=changes).

Add clearOnUniverseChange: on a runtime msg.universe switch, optionally emit
a full zeroed universe for the new universe until real data arrives.

The full universe state is now always cached internally (needed for the
keepalive and the blanking), the keepalive timer is cleared on close, and
the interval field is shown only for the interval trigger. Rebuild dist.