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/size.rs

@@ -0,0 +1,44 @@
+//! Integer widths and safe logical offsets used by terminal pages.
+use std::marker::PhantomData;
+/// Maximum addressable page size.
+pub const MAX_PAGE_SIZE: u32 = u32::MAX;
+/// Byte offset within a page.
+pub type OffsetInt = u32;
+/// Number of cells in a grid dimension.
+pub type CellCountInt = u16;
+/// Style capacity and identifiers.
+pub type StyleCountInt = u16;
+/// Hyperlink capacity and identifiers.
+pub type HyperlinkCountInt = u16;
+/// Bytes of grapheme storage.
+pub type GraphemeBytesInt = u32;
+/// Bytes of string storage.
+pub type StringBytesInt = u32;
+/// A typed logical byte offset; never converted to a raw pointer.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+#[repr(transparent)]
+pub struct Offset<T> {
+    offset: u32,
+    marker: PhantomData<fn() -> T>,
+}
+impl<T> Offset<T> {
+    /// Construct a logical byte offset.
+    pub const fn new(offset: u32) -> Self {
+        Self {
+            offset,
+            marker: PhantomData,
+        }
+    }
+    /// Return the byte displacement.
+    pub const fn offset(self) -> u32 {
+        self.offset
+    }
+}
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn offset() {
+        assert_eq!(std::mem::size_of::<Offset<u8>>(), 4);
+    }
+}