[puppeteer] Add expression preview strip for selected character

[S1mplector]

Module: Puppeteer

Goal:
Make it easy to preview a selected character's expressions from inside the animation viewport.

Acceptance criteria:

• Selection panel or viewport overlay shows compact expression options for the selected character.
• Choosing an expression updates the preview without changing keyframes unless explicitly applied.
• Current expression is clearly indicated.
• Control remains usable for characters with many expressions.

Notes:
This is a workflow improvement for authoring, not a replacement for script expression commands.