codeSplitter: scenes follow-ups (browser check, keyboard queue, finer scenes) #1610

Left over from #1587 / PR #1609.

• MANUAL RUN: watch play_scene / scene_step and the shift-click queue in a real browser (cursor, held steps, skip, reduced motion). Only unit tests and a syntax parse ran so far.
• Queue has no keyboard or touch route: it is shift-click on a chapter icon only. Add a visible "queue" control.
• Scenes are exactly the tour's six chapters. Let a scene be any slice of steps, and let play_scene take one, so agents can learn single features.
• scene_step on a repo-page step needs the file page open first; play_scene could open it itself when given owner/repo.
• Palette commands for scenes (the tools exist only over WebMCP today).