Muster/T2R6 should declare and verify broker adapter dependencies

[azide0x37]

A T2R6 Home Assistant MQTT bridge implementation exposed a gap in the Muster lifecycle contract.

Situation:

• The appliance had Home Assistant MQTT publishing enabled.
• The bridge was configured to use mosquitto_pub as its real broker adapter.
• The systemd service failed every timer run because the adapter binary was not installed.
• Mock-first outbox artifacts were generated correctly, but no discovery reached Home Assistant until the OS package providing the MQTT client was installed.

What was missing:

• The implementation had a runtime dependency on the configured broker adapter, but the install/doctor/package contract did not make that dependency explicit.
• T2R6 says discovery/state/control traffic should be mockable first and broker plumbing added later, but real-broker mode needs concrete dependency handling.

Expected Muster behavior:

• Patterns that declare real adapters such as real_broker_adapter: mosquitto_pub should drive package/dependency documentation.
• Installer should install or clearly skip required OS packages using an explicit opt-out, similar to the published dvd-ingester installer pattern.
• doctor.sh should fail with a precise dependency message when real MQTT publishing is enabled and the configured adapter is unavailable.
• README self-certification should include dependency evidence for non-shell runtime adapters.
• T2R6 reference artifacts should include a mosquitto-clients dependency or a generic adapter dependency slot.

Recommendation:
Add a Muster contract check for adapter dependencies, probably under C6 lifecycle plus T2R6-specific requirements, and update the reference artifacts so this failure is caught before deployment.