The cached _estimateLineHeight in TextLayoutManager was never invalidated
when the font changed. The vertical cursor movement calculation uses this
estimate to compute the target y-coordinate, and after enough font size
increases the stale (too small) value prevented moveDown: from crossing
into the next line. Up arrow was unaffected because subtracting from the
line top always lands in the previous line.

Fix: re-assign renderDelegate after font/lineHeight changes to trigger
its didSet which clears the cached estimate. Also handle arrow keys
explicitly in the event monitor and add Ctrl+N/P (moveDown/moveUp)
to handleCommand for robustness.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- Extract renderDelegate cache invalidation into refreshEstimatedLineHeightCache() helper
- Normalize modifier flags to [shift, control, option, command] so Ctrl-N/P work with Caps Lock/Fn
- Fix handleArrowKey doc comment to accurately describe supported modifier combinations
- Centralize key code constants (tab, downArrow, upArrow) as static properties

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>