refactor(tui): extract _select_origin_widget helper

[EthanGuo-coder]

The previous head was based on fc6b5ad, which is no longer in main's history after the recent force-push, so this PR was rendering a bogus 250-commit / 100-file diff against an ancient merge base. Re-applied cleanly on top of 4086d5c. The test file the original description mentioned has been dropped, since the tree carries no test suite and no pytest dependency.

Changes:

• Extract the inline Textual 8.x / ≤7.x branch in _patch_auto_scroll_for_selection into a module-level _select_origin_widget(screen) helper at the top of the # ---------- App ---------- section. The call site becomes a one-liner.
• Guard .start before dereferencing it. This is future-proofing rather than a fix for a currently reachable state: SelectState.start is non-optional in Textual 8.2.6, so the existing select_state.start.container cannot raise on any shape Textual produces today. The guard only means that a future private-API change degrades to "no extra auto-scroll candidate" instead of an AttributeError raised inside a mouse-move handler.

No behaviour change on any input Textual produces today. Verified by running the current inline implementation and the helper side by side over a 20-case matrix covering every presence / absence / None combination of _select_state, _select_state.start and _select_start: 12 cases identical, and the only 8 differences are exactly the guard described above — the inline version raises AttributeError, the helper returns None. An empty _select_start tuple still raises IndexError in both, unchanged.

Refs #373, #379