feat: fire OnScrollback hook for lines scrolled off the main screen

[leogr]

OnScrollback is already public and documented ("called every time a line is about to be pushed out of the visible screen region"), but the implementation in scrollUpN is a no-op - the call was commented out, so setting the hook does nothing today.

This PR wires it up: when the main screen scrolls a line off the top, the hook fires with that line (content + per-cell format).

A few details:

• It only fires for history-producing scrolls - the main screen (not the alt screen, which has no scrollback) with a full-screen scroll region. Partial-region scrolls don't accumulate history.
• Each line is deep-copied before the scroll (since scrollUp recycles the row slices), and the hook fires afterward, from the stable post-scroll state.
• The per-cell Format is rebuilt from the canvas regions, so the hook gets the same Line shape that Render produces.

I also tightened the doc to match this contract - including that the hook runs synchronously during input and shouldn't re-enter the terminal.

[vito]

Woops, just needs a rebase now that #5 is merged. (Thanks!)

[leogr]

Woops, just needs a rebase now that #5 is merged. (Thanks!)

Done!