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)

Cargo.lock

@@ -872,6 +872,7 @@ dependencies = [
  "ghostty-os",
  "log",
  "ratatui",
+ "tempfile",
  "thiserror 2.0.21",
 ]
 
@@ -916,6 +917,7 @@ version = "0.1.0"
 dependencies = [
  "bitflags 2.13.2",
  "freetype-sys",
+ "ghostty-datastruct",
  "ghostty-unicode",
  "harfbuzz-sys",
  "log",