Restore RichLog follow-state parity and expand reflow behavior

Normalized profile

What this task asks for

Restore RichLog follow-mode parity with Log and preserve expand reflow behavior for justified writes. Expose `is_following_end`, `follow_end()`, and a `FollowChanged` message on Log and RichLog. Post follow changes only when the following state boolean changes. Follow appended content only while already at the end and restore following after scrolling back to the end. Keep the viewport stable during appends and `max_lines` pruning when not following. Preserve expanded and justified RichLog rendering across deferred writes and resizes. Add an interactive example for toggling follow state, appending entries, and recording follow events.