simulate(tui): unify up/down navigation

[u9g]

What

Simplifies keyboard navigation in the lk agent simulate running view.

Previously the view had two overlapping navigation models: i/k moved a cursor over the job list while the arrow keys scrolled the whole page, and the summary/logs at the bottom weren't reachable by the cursor at all — so which key did what depended on where you were looking.

This collapses everything into a single motion:

• Up/Down move the cursor through the job list and spill into page scrolling at each end, so the header/description above and the summary/logs below come into view continuously. The whole view is now fully scrollable with just the arrows (the mouse wheel maps to these in alt-screen).
• Right / Enter open the selected job's detail.
• Left / Esc go back.
• i / j / k / l are removed.

Detail view, expanded-description, and PgUp/PgDn (fast page/log scroll) are unchanged.

Implementation

• New navVertical(delta) — cursor movement with end-of-list scroll spill, reusing the existing followCursor/cursorViewLine machinery to keep the cursor visible while it drives and pin the viewport once you scroll past an end. It reads the last render's window height via a new pageWinRows field.
• Removed the scrollBy/moveCursor split; cursor movement no longer wraps, so hitting the last job triggers the spill.
• Hint bar updated accordingly.

Testing

go build ./... and the simulate/TUI tests pass. Per the repo's no-TUI-output-test convention, no assertions on rendered output were added — verified by hand against a finished run.