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/cursor.rs

@@ -0,0 +1,22 @@
+//! Cursor visual style (ZIG/src/terminal/cursor.zig).
+
+/// The visual style of the cursor. Whether or not it blinks
+/// is determined by mode 12 (modes.zig). This mode is synchronized
+/// with CSI q, the same as xterm.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum Style {
+    /// DECSCUSR 5, 6
+    Bar,
+    /// DECSCUSR 1, 2
+    #[default]
+    Block,
+    /// DECSCUSR 3, 4
+    Underline,
+
+    /// The cursor styles below aren't known by DESCUSR and are custom
+    /// implemented in Ghostty. They are reported as some standard style
+    /// if requested, though.
+    /// Hollow block cursor. This is a block cursor with the center empty.
+    /// Reported as DECSCUSR 1 or 2 (block).
+    BlockHollow,
+}