native UXP audio fading transition support

Premiere Pro 26.5.2 (UXP plugins), macOS

SUMMARY
UXP plugins can read and add video transitions, but they can't do anything with audio transitions. Plugins can't see them, create them, remove them, or carry them along when copying clips. That makes it impossible to build tools that rebuild or move an edit "exactly as it was", because almost every dialogue edit uses short audio fades to avoid pops.

USE CASE
I convert finished vertical shorts into horizontal long-form videos with a UXP panel. The panel copies each finished edit onto a master timeline. Clips, effects and keyframes come across through SequenceEditor.createCloneTrackItemAction, but the audio crossfades are lost. One edit can have around 50 of them, usually 2–8 frames long. My only workaround is to place each edit as a nested sequence and un-nest it with a third-party CEP extension. That depends on CEP and ExtendScript, which are being phased out.

WHAT HAPPENS TODAY (UXP)

• audioTrack.getTrackItems(TrackItemType.TRANSITION, false) returns one null per audio transition. There is no start, end, duration, type or alignment.
• There is no way to add an audio transition. Video clips have createAddVideoTransitionAction; audio clips have no equivalent.
• createCloneTrackItemAction copies clips (also into another sequence), but not the transitions attached to them.
• There is no "un-nest" (expand a nested sequence in place) action.

WHAT WE NEED

1. Read audio transitions. Return real objects from audioTrack.getTrackItems(TRANSITION) with: start and end time, duration, alignment (start of cut / center / end of cut), the transition type (match name, e.g. Constant Power, Constant Gain, Exponential Fade), and the clip(s) it belongs to.
2. Add audio transitions. An action like createAddAudioTransitionAction on audio clip track items, mirroring the video one: the transition type (from a factory, e.g. by match name), duration, alignment, and whether it goes on the clip's start or end.
3. Change and remove audio transitions: set the duration and alignment, and remove one, as actions usable inside project.executeTransaction (so they undo in one step).
4. Nice to have:
   a) an option on createCloneTrackItemAction to copy the clip's transitions with it;
   b) an "expand nested sequence in place" (un-nest) action that keeps clips, effects, keyframes and both video and audio transitions.

WHY IT MATTERS
CEP and ExtendScript (QE) can already do this, and tools built on them do. As CEP goes away, UXP needs the same ability, or automation for dialogue-heavy editing (assembly, conforming, un-nesting, reformatting for other aspect ratios) can't reproduce an editor's audio work.