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

Cargo.lock

@@ -88,12 +88,6 @@ dependencies = [
  "num-traits",
 ]
 
-[[package]]
-name = "arrayref"
-version = "0.3.9"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "76a2e8124351fda1ef8aaaa3bbd7ebbcb486bbcd4225aca0aa0d84bb2db8fecb"
-
 [[package]]
 name = "arrayvec"
 version = "0.7.8"
@@ -1549,7 +1543,6 @@ dependencies = [
  "objc2-foundation",
  "png",
  "thiserror 2.0.21",
- "tiny-skia",
  "yeslogic-fontconfig-sys",
 ]
 
@@ -3861,12 +3854,6 @@ version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a2eb9349b6444b326872e140eb1cf5e7c522154d69e7a0ffb0fb81c06b37543f"
 
-[[package]]
-name = "strict-num"
-version = "0.1.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6637bab7722d379c8b41ba849228d680cc12d0a45ba1fa2b48f2a30577a06731"
-
 [[package]]
 name = "strsim"
 version = "0.11.1"
@@ -4148,32 +4135,6 @@ dependencies = [
  "time-core",
 ]
 
-[[package]]
-name = "tiny-skia"
-version = "0.12.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "47ffee5eaaf5527f630fb0e356b90ebdec84d5d18d937c5e440350f88c5a91ea"
-dependencies = [
- "arrayref",
- "arrayvec",
- "bytemuck",
- "cfg-if",
- "log",
- "png",
- "tiny-skia-path",
-]
-
-[[package]]
-name = "tiny-skia-path"
-version = "0.12.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "edca365c3faccca67d06593c5980fa6c57687de727a03131735bb85f01fdeeb9"
-dependencies = [
- "arrayref",
- "bytemuck",
- "strict-num",
-]
-
 [[package]]
 name = "toml"
 version = "1.1.8+spec-1.1.0"