Pin the activity log to its newest entry

[iTerminate]

Summary

• The docked activity log ticker follows the newest entry across reloads, scope/page changes, and live appends — unless the reader has scrolled up, in which case it stays put.
• The maximized selectable list gets the same rule: a live append moves the highlight to the new newest entry only when it was already parked there.

Root cause

Every reload/reflow path does Log.clear() followed by write_lines. clear() zeroes virtual_size but leaves scroll_y untouched, so the next write's is_vertical_scroll_end check reads stale and Textual's own auto_scroll silently declines to scroll — including for every live append after that, since nothing ever puts scroll_y back at the bottom on its own.

Fixes #44.

Test plan

• tests/test_log_scroll.py (new) — pinned/scrolled-up behavior across open, live append, reload, empty history, and reflow
• tests/test_log_maximize.py — added a follow-the-newest-selection case alongside the existing preserves-selection case
• uv run ruff check . / uv run pyright clean
• Full pytest suite: 1306 passed