Add mDNS/Bonjour discovery for remote apps (no hard-coded ports)

[dccote]

Summary

RemoteControllable apps can now publish themselves on the local network and be
found by name, so clients no longer need a hard-coded host/port. Built on the
existing XML-RPC remote layer; the transport is unchanged.

What's new

Server — RemoteControllable.advertise_remote(port=0, host="0.0.0.0", service_type="_mytk._tcp.local.", addr=None)
starts the remote server on an OS-picked free port and announces it over mDNS.
app_name becomes the service instance name and is copied into the TXT record.
stop_remote() now also withdraws the announcement.

Client — mytk.discover(app_name=None, service_type="_mytk._tcp.local.", timeout=3.0)
browses for an advertised server and returns a proxy through the existing
connect(), so RemoteAppMismatch verification still applies. app_name
filters by the advertised identity.

CLI — mytk-remote --discover locates the server over mDNS instead of using
--host/--port; --app-name selects which app, --service-type/--timeout
tune the browse. Explicit --host/--port behavior is unchanged.

Usage

app.advertise_remote()                 # free port + mDNS announcement
far = mytk.discover(app_name="Microscope")
far.turn_on()

mytk-remote --discover --list
mytk-remote --discover --app-name Microscope "status()"

Packaging

New optional extra mytk[remote] (pulls in zeroconf). advertise_remote()
installs it on demand via ModulesManager (Tk context); discover() imports it
directly and raises a clear ImportError if missing.

Design notes

• Service type describes the application protocol (_mytk._tcp), not the
  transport, per DNS-SD; multiple apps share it and are told apart by
  instance name / app_name.
• Default host="0.0.0.0" on advertise_remote() makes the service reachable
  from other machines (mDNS is a LAN feature); pass 127.0.0.1 to keep it
  same-machine only. start_remote() still defaults to localhost.

Tests

mytk/tests/testRemoteDiscovery.py covers discover() browsing, app_name
filtering, timeout, and the missing-zeroconf ImportError, plus the CLI
--discover wiring. Hermetic via a fake zeroconf module — no multicast, no
sockets, no Tk window.

Compatibility

Backward compatible: no changes to existing signatures/defaults of
start_remote, connect, or the CLI's --host/--port path. zeroconf is
only needed when discovery is used.

🤖 Generated with Claude Code