fix: pixel-identical rendering (z2d stroker port) and last P3 items

Resolves the 3 P3 items in docs/rewrite/notes/review-rv12-codex.md
(final-Z15.md):
- Sprite strokes: tiny-skia is replaced by a port of z2d 0.10's stroke
  plotter (sprite/stroke.rs) plus z2d's arc segmentation and truncating
  integer compositing, feeding the ported 4x4 multisample rasterizer.
  All 36 reference sprite sheets now match Ghostty's byte for byte (test
  tolerance 0), and `just visual-parity` with a production build shows
  0 differing pixels (max delta 0) against the installed Ghostty 1.3.1
  for the test page (block and bar cursor) and the tabs+splits scenario,
  terminal area and chrome alike. tiny-skia dependency removed.
- zig_error maps errno values per target via libc (Linux, Darwin/BSD)
  and Win32 codes the way Zig std does on Windows.
- theme=. / theme=.. report "it is a directory" via a port of
  std.fs.path.basename, matching Ghostty.

Test Plan:
- cargo test --workspace --no-fail-fast (3498 passed, exit 0)
- cargo build --profile production && just visual-parity
- just check-macos (both), just check-windows, just check-wasm

crates/ghostty-font/Cargo.toml

@@ -6,7 +6,6 @@ license.workspace = true
 
 [dependencies]
 ghostty-os.workspace = true
-tiny-skia = "0.12.0"
 png = "0.18.1"
 thiserror = "2.0.21"
 log = "0.4.34"