CarouselView with SelectionArea edge-scrolls (snaps items) during mouse text selection #190737

Follow-up to #149426 and PR #189544.

PR #189544 stops paged scrollables from auto-scrolling while a SelectionArea extends a selection with the mouse, by overriding the page-snapping flag on PageScrollPhysics.

CarouselScrollPhysics (in packages/flutter/lib/src/material/carousel.dart) also snaps to item boundaries, but it extends ScrollPhysics directly rather than PageScrollPhysics, so it does not inherit that override. A CarouselView wrapped in a SelectionArea should therefore show the same problem: drag-selecting text with a mouse past the edge snaps to the next/previous item instead of extending the selection.

Expected: mouse selection-drag extends the selection; the carousel does not snap items.

Fix direction: once the new page-snapping property lands on ScrollPhysics via #189544, override it in CarouselScrollPhysics. It lives in Material, hence a separate issue from the widgets-layer fix.

Suggested labels: framework, f: scrolling, f: material design, a: text input