feat(terminal): port Screen, ScreenSet, Tabstops and Selection

Ports ZIG/src/terminal/{Screen,ScreenSet,Tabstops,Selection}.zig on top
of the page/PageList layer. The cursor holds a TrackedPin; Zig's cached
page_row/page_cell pointers become accessor methods. Selection methods
take the PageList explicitly because tracked bounds are handles.

Screen tests use minimal plain-text dump/selection-string helpers that
follow the formatter's semantics; they will delegate to the real
formatter once formatter.zig is ported.

Gaps found in the PageList layer are recorded for the parity pass in
docs/rewrite/notes/ghostty-terminal-pagelist-requests.md; three resize
tests are #[ignore]d pending those fixes (PageList::resize keeps
trailing blank rows Zig drops). Kitty image storage and StringMap are
placeholders.

Test Plan:
- cargo test -p ghostty-terminal (443 passed, 3 ignored; Screen 183/184,
  Selection 18/18, Tabstops 4/5 - skipped tests use allocator failure
  injection)

crates/ghostty-terminal/src/lib.rs

@@ -1,9 +1,15 @@
 //! Safe page-based terminal memory model.
 #![cfg_attr(test, allow(clippy::unwrap_used))]
+pub mod cursor;
 pub mod hyperlink;
 pub mod page;
 pub mod page_list;
+pub mod pin_iter;
 pub mod point;
 pub mod ref_counted_set;
+pub mod screen;
+pub mod screen_set;
+pub mod selection;
 pub mod size;
 pub mod style;
+pub mod tabstops;