Auto-scroll when a drag-selection reaches the top/bottom edge

[bastien-gallay]

What

While drag-selecting, moving the pointer to the top or bottom edge of the
terminal should scroll the viewport and keep extending the selection —
standard select-past-the-edge behaviour.

Current state

Dragging past the canvas edge simply stops updating the selection head:
cell_at() uses cursor.position_in(bounds) which returns None off-bounds
(crates/app/src/shell/terminal.rs:212-217, 360), so head freezes at the
last in-bounds cell. There is no edge detection and no autoscroll tick.

Fix direction

• Detect the pointer at/beyond the top/bottom edge during an active selection.
• Drive a scroll tick (and extend head) while held there.
• Shares the absolute-line anchoring work with the "selection follows
  scroll" issue — both need selection off viewport coordinates.

[bastien-gallay]

Refactor prerequisite: #168 (C1 terminal/selection.rs) — the whole selection wave (#156-#159) edits selection_span/ordered/cell_at; do the split first.