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/hyperlink.rs

@@ -0,0 +1,43 @@
+//! Owned OSC 8 hyperlinks and per-page identifiers.
+/// Nonzero page-local hyperlink identifier.
+pub type Id = u16;
+/// Mapping from a cell index to its hyperlink.
+pub type Map = std::collections::HashMap<usize, Id>;
+/// An explicit OSC 8 ID or an automatically generated screen-local ID.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub enum HyperlinkId {
+    /// Application-provided byte string.
+    Explicit(Vec<u8>),
+    /// Screen-generated identifier.
+    Implicit(u32),
+}
+/// A decoded hyperlink owning its ID and URI.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct Hyperlink {
+    /// Hyperlink identity.
+    pub id: HyperlinkId,
+    /// URI bytes, which need not be UTF-8.
+    pub uri: Vec<u8>,
+}
+impl Hyperlink {
+    /// Duplicate the owned strings.
+    pub fn dupe(&self) -> Self {
+        self.clone()
+    }
+    /// Compare identity and URI.
+    pub fn eql(&self, other: &Self) -> bool {
+        self == other
+    }
+    /// Logical string allocation including 32-byte chunk rounding.
+    pub fn string_bytes(&self) -> usize {
+        self.uri.len().next_multiple_of(32)
+            + match &self.id {
+                HyperlinkId::Explicit(v) => v.len().next_multiple_of(32),
+                HyperlinkId::Implicit(_) => 0,
+            }
+    }
+}
+/// Owned page entry; unlike Zig it needs no base address for decoding.
+pub type PageEntry = Hyperlink;
+/// Reference-counted set of page entries.
+pub type Set = crate::ref_counted_set::RefCountedSet<PageEntry>;