feat: visual parity with the original Ghostty (pixel-diff pipeline)

Adds scripts/visual-parity.sh (`just visual-parity`): launches the
installed Ghostty 1.3.1 and the Rust build in an isolated headless
nested weston (GL renderer, Mesa forced, weston-screenshooter via
--debug), with identical isolated config, a deterministic test page
(ASCII, ligatures, all SGR styles and underline kinds, 16/256/truecolor,
box/block/braille/powerline/nerd icons, CJK, emoji, wide chars,
cursor) and a tabs+splits scenario driven through Xwayland, then diffs
the captures (terminal area and chrome separately, heatmaps).

Fixes found by comparing against the original:
- A pane stayed blank after a nested split: the GTK render path called
  draw_frame(false) where Ghostty uses drawFrame(true) (a re-realized
  GLArea has a new swap chain that nothing else redraws).
- Sprite glyph antialiasing now ports z2d's default 4x4 multisample
  rasterizer instead of tiny-skia's analytic coverage (pixels differing
  from Ghostty's reference sprite sheets: 5.76% -> 1.20%, none off by
  more than 96 levels); the sheet test tolerance is tightened to 0.1%.

Result vs Ghostty 1.3.1: test page 0.020% of pixels differ (196 px at
curly-underline ends and diagonal box strokes, max delta 86 levels);
tabs+splits and headerbar/tab bar are bit-identical. Details in
docs/rewrite/notes/visual-parity-VP.md.

Test Plan:
- just visual-parity (needs weston, weston-screenshooter, xdotool)
- cargo test -p ghostty-font (sprite sheet comparison)

justfile

@@ -290,3 +290,7 @@ check-wasm:
 check-all: clippy test check-wasm check-windows
     scripts/macos-crosscheck.sh clippy aarch64-apple-darwin
     scripts/macos-crosscheck.sh clippy x86_64-apple-darwin
+
+# Capture original Ghostty vs the Rust build in a headless weston and diff them.
+visual-parity *scenarios:
+    RUST_BIN="${RUST_BIN:-target/vp/production/ghostty}" scripts/visual-parity.sh {{scenarios}}