Light transition buffers: group members never applied, zero-level vs on_off ordering #988

Problem

Two edge cases around the light transition buffers, found while reviewing the follow-up to #767 (buffering on_off reports during a transition).

• Group members buffer on_off / brightness reports while _transitioning_group is set, but async_transition_complete on the group only calls transition_off() on the members. Nothing applies a member's buffer, and the debounced member refresh does not poll when polling is disabled, so a member can keep stale state until its next report. This already applied to the brightness buffer before the follow-up.
• In async_transition_complete, the on_off buffer is applied after the brightness buffer. An on_off=True report followed by a current_level=0 report in the same window now ends up on, where the zero level alone would have ended off.

Expected

A member's buffered reports are applied (or dropped deliberately) when the group transition ends, and the ordering between a zero-level report and an on_off report is decided on purpose, ideally by arrival order.

Context

Both are rare. Related: #734 (light class cleanup), #767.