feat(terminal): port page memory model (partial PageList)

First cut of the terminal core memory model from ZIG/src/terminal:
size, point, style, hyperlink, ref_counted_set, page and PageList.

Design (see docs/rewrite/ARCHITECTURE.md decisions 2-4):
- Cell/Row are 8-byte u64 wrappers with Zig's exact bit offsets.
- Page owns row/cell Vecs plus style set, grapheme and hyperlink maps;
  Page::layout reproduces Ghostty's byte-size formula on 64-bit Linux so
  scrollback-limit pruning matches.
- PageList keeps nodes in a generation-checked slab with a free list and
  list-owned tracked pins addressed by handles.

This is explicitly partial (docs/rewrite/notes/ghostty-terminal-
pagelist.md): 151/202 PageList tests ported; reflow rebuilds a snapshot
instead of Zig's incremental ReflowCursor; point<->pin conversion walks
from the list head (no active/viewport caches); RefCountedSet doesn't
reproduce probe-dependent ID reuse. A follow-up pass finishes parity.

Test Plan:
- cargo test -p ghostty-terminal (239 tests)

crates/ghostty-terminal/src/point.rs

@@ -0,0 +1,65 @@
+//! Coordinates relative to the active area, viewport, screen, or history.
+/// Reference frame for a terminal coordinate.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub enum Tag {
+    /// Editable bottommost screen area, including unwritten rows.
+    Active,
+    /// Visible area, including user scrolling.
+    Viewport,
+    /// Entire screen including scrollback.
+    Screen,
+    /// Scrollback preceding the active area.
+    History,
+}
+/// An x/y coordinate; history may exceed a single page's row count.
+#[derive(Debug, Default, Copy, Clone, PartialEq, Eq, Hash)]
+pub struct Coordinate {
+    /// Column.
+    pub x: u16,
+    /// Row relative to the point's reference frame.
+    pub y: u32,
+}
+impl Coordinate {
+    /// Compare both coordinates.
+    pub fn eql(self, other: Self) -> bool {
+        self == other
+    }
+}
+/// A coordinate with its reference frame.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub enum Point {
+    /// Editable screen coordinate.
+    Active(Coordinate),
+    /// Visible viewport coordinate.
+    Viewport(Coordinate),
+    /// Whole-screen coordinate.
+    Screen(Coordinate),
+    /// Scrollback coordinate.
+    History(Coordinate),
+}
+impl Point {
+    /// Construct a point in a given frame.
+    pub const fn new(tag: Tag, coord: Coordinate) -> Self {
+        match tag {
+            Tag::Active => Self::Active(coord),
+            Tag::Viewport => Self::Viewport(coord),
+            Tag::Screen => Self::Screen(coord),
+            Tag::History => Self::History(coord),
+        }
+    }
+    /// Extract its coordinate.
+    pub const fn coord(self) -> Coordinate {
+        match self {
+            Self::Active(v) | Self::Viewport(v) | Self::Screen(v) | Self::History(v) => v,
+        }
+    }
+    /// Extract its reference frame.
+    pub const fn tag(self) -> Tag {
+        match self {
+            Self::Active(_) => Tag::Active,
+            Self::Viewport(_) => Tag::Viewport,
+            Self::Screen(_) => Tag::Screen,
+            Self::History(_) => Tag::History,
+        }
+    }
+}