Fix teleprompter-to-editor cursor mapping, add return-to-cursor flow, and eliminate editor lag while CueMode stays open

[hddevteam]

Summary

CueMode currently has three related usability/performance issues in teleprompter mode:

1. Double-click → editor cursor is inaccurate

   • When users double-click text in CueMode to return to the editor, the editor cursor/selection often does not land near the clicked text.
   • This is especially visible for rendered Markdown, where the current implementation uses rendered block indices and plain-text snippets instead of a robust source position mapping strategy.

2. Need a way to return CueMode to the current editor cursor location

   • After jumping from CueMode back into the editor for inspection or editing, users need a dedicated way to move the teleprompter view back to the current editor cursor location so they can continue reading from the right place.

3. Editing lag when CueMode tab remains open

   • If the CueMode tab stays open while users edit the source document, typing can become laggy or even temporarily unresponsive.
   • The current source-document sync path appears to re-render too aggressively.

Proposed TDD scope

• Add failing tests that reproduce:
  • inaccurate editor selection after double-click from CueMode
  • ability to reveal current editor cursor location in CueMode
  • excessive / full-page refresh behavior while CueMode remains open during editing
• Implement a more robust position resolution strategy for teleprompter → editor navigation.
• Add a dedicated CueMode-side/editor-side flow to reveal the current editor cursor in the teleprompter.
• Refactor sync updates to reduce heavy re-rendering and avoid typing lag.

Acceptance criteria

• Double-clicking visible text in CueMode opens the editor with the cursor/selection landing on or very near the clicked source text.
• Users can explicitly return CueMode to the current editor cursor location.
• Keeping CueMode open while editing no longer causes noticeable typing lag or input blocking.
• New automated tests cover these behaviors.

Notes

Implementation should follow TDD and preserve existing CueMode / Presentation Mode behavior.