clamp_selection

Return selection shifted past a diff row's gutter, if one is set.

Every form a selection can take over a single-line row covers the gutter unless its start is moved past it:

• Selection(None, None) — the row sits mid-selection. Textual extracts the row's full text, so the start must move to the gutter's end even though no endpoint lands here.
• Selection(None, end) — entered from above; same move, plus an end still inside the gutter means nothing selectable is covered, reported as None so the row drops out of the screen's selection map.
• Selection(start, None) / Selection(start, end) — pull any endpoint inside the gutter forward to its end; a range that then collapses (wholly gutter) is None.

Endpoints arrive as offsets into the row's logical content, not as visual screen coordinates, because every strip carries its own offset metadata. A wrapped row's continuations therefore need no rule of their own: their offsets — including the ones their synthetic gutter maps to — all land at or past prefix_len, so the gutter tests below cannot fire on them.