Add T2R6 Home Assistant MQTT bridge pattern

[azide0x37]

Summary

• Add T2R6.home-assistant-mqtt-bridge to the Muster Pattern Library with install, rollback, uninstall, doctor, unit, and test coverage.
• Document the new pattern in the library index, graph, completion docs, and production-beta contract.
• Add focused bridge tests plus completion and inventory updates so the new pattern is rendered and validated correctly.

Testing

• Pattern validation and composition checks passed.
• Production-beta and completion rendering checks passed.
• Unit tests for the new bridge pattern passed.