Pin each MusicMouse host to a deploy branch, and freeze musicmouse

musicmouse_version pointed both hosts at rearchitect-backend, a branch that only
ever existed on a laptop - so the previous commit's rework could never actually
have run. Point each host at a deploy/<host> branch on the remote instead. Those
branches are the deployment record: what is installed on that box right now,
moved with a force-push when you mean to deploy.

musicmouse (Kinderzimmer) is the reason this matters. It is in daily use and
still runs the pre-rearchitecture backend out of /opt/musicmouse_venv and
espmusicmouse/host_driver/main.py. That commit has no python-backend/
pyproject.toml, no python-backend/musicmouse.service and no web/ at all, so this
role cannot install it - and a run would have force-checked-out incompatible
code over a working device first and failed afterwards. Set
mediapi_install_kidsmusic: false there and leave it alone;
deploy/musicmouse records the commit that is installed, for rollback. Add a
stat + fail right after the checkout so the same mistake stops with an
explanation instead of failing three tasks deeper.

Also in the role:

- Install the typing game's curriculum next to config.yml. A config with a
  tippen: section refuses to start without it - curriculum_file is validated as
  must-exist - so musicdolphin's new config needs it present.

- Stop clobbering config.yml. The app writes to it at runtime: parent mode
  patches the volume keys and the remote-control page rewrites the remote:
  block. A plain copy: undid that on every run. Install-once by default, with
  -e pi_musicmouse_force_config=true to push a change deliberately.

- Fill out config-musicdolphin.yml against the current schema: library root and
  cache, an absolute static_dir (../web/dist would resolve to /media/web/dist
  from /media/musicmouse and silently serve nothing), tippen, and the volume
  range for a screen-driven instance. ha: is left commented out until the
  Wohnzimmer-oben entity ids are picked - the ones in the dev config are
  Arbeitszimmer. No mqtt: (it would publish a device of dead entities on a
  simulate-only host) and no lirc:/remote: (this host runs irserver, a serial
  daemon, not the lircd this speaks TCP to).

The curriculum is a verbatim copy of app content, so exclude it from
ansible-lint rather than reformat it away from its source.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

host_vars/musicdolphin.yml

@@ -15,4 +15,4 @@ mediapi_has_dhtsensor: true
 # via pi_kiosk instead. See roles/pi_musicmouse/files/config-musicdolphin.yml.
 mediapi_install_kidsmusic: true
 mediapi_has_monitor: true
-musicmouse_version: rearchitect-backend
+musicmouse_version: deploy/musicdolphin