fixt the Scrolling back to the bottom does not restore anchored scroll

[prabinKh]

1. Smarter anchor handling during scroll (widget.py)

Added _handle_scroll_anchor() and use it in scroll_to / _scroll_to instead of unconditionally calling release_anchor():

Release the anchor only when scrolling away from the bottom.
Re-engage the anchor when the scroll target reaches the bottom (target_y >= max_scroll_y).
2. More reliable anchor check (widget.py)

Updated _check_anchor() to use is_vertical_scroll_end instead of a raw scroll_y >= max_scroll_y comparison.

3. Re-check anchor during layout (_compositor.py)

During layout, if a widget is anchored but released and scroll_y is still at the bottom edge, re-engage the anchor before applying bottom-gluing. This handles the case where content grows while the user is already at the bottom.

Test plan
Added tests/test_anchor_scroll.py with 4 tests:
Anchored widget follows new content
Scrolling up releases the anchor
Scrolling back to bottom re-engages the anchor
Anchor stays engaged while content streams after restore
Ran pytest tests/test_anchor_scroll.py -v — all 4 passed
Ran pytest tests/test_compositor.py tests/test_containers.py — no regressions
Files changed
src/textual/widget.py — anchor release/restore logic during scroll
src/textual/_compositor.py — re-check anchor during layout
tests/test_anchor_scroll.py — new test coverage