…intent

Three related improvements to output rendering:

1. Debounce concurrent render_full_session calls

   Problem: Rapid session switches trigger multiple redundant renders.
   Solution: Coalesce into one in-flight render + trailing render with
   latest options.

2. Refactor scroll_to_bottom conditions
   Convert nested if-else chain to declarative condition table for
   better readability and maintainability.

3. Add WinScrolled autocmd
   Track user scroll position to accurately detect scroll intent
   (whether user is at bottom or has scrolled up).