[P3] Scroll container

[fissible]

A reusable scrollable viewport. Scrolling must NOT be baked into individual widgets.

Needs:

• vertical scrolling
• optional horizontal scrolling
• clipping rendered content to viewport bounds
• scroll offset tracking
• page up/down, home/end
• keep-selected-item-visible behavior (scroll to show cursor)

Deliverable: src/scroll.sh with shellframe_scroll_* functions.
Effort: L (~1 day) | Phase: 3 | Deps: P2 clipping helpers