feat(terminal): port kitty graphics protocol storage and execution

Ports ZIG/src/terminal/kitty/graphics_*.zig:
- image loading for direct, file, temp-file and shared-memory
  transmissions (rustix::shm, no unsafe), chunking, zlib and PNG decode
- image storage with placements (incl. virtual) tracked by PageList pins,
  all delete modes, byte-limit eviction and dirty tracking
- unicode placeholder decoding and placement iteration
- command execution, generic over an ExecTerminal trait

Deviations: storage methods take the PageList explicitly; evicted or
replaced placements now release their tracked pins (Zig leaks them);
file transmissions with S= truncate per the kitty spec. Zig quirks kept
for parity: delete-range matches all placements, eviction stops only
after freeing strictly more than requested, set_limit(0) resets.

Not yet wired: Screen still holds the KittyImages placeholder, so
Terminal does not implement ExecTerminal; integration steps are in
docs/rewrite/notes/ghostty-terminal-kitty.md.

Test Plan:
- cargo test -p ghostty-terminal (66 kitty tests; all Zig tests ported)

crates/ghostty-terminal/src/lib.rs

@@ -4,6 +4,7 @@ pub mod cursor;
 pub mod formatter;
 pub mod highlight;
 pub mod hyperlink;
+pub mod kitty;
 pub mod page;
 pub mod page_list;
 pub mod pin_iter;