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)

scripts/visual-parity/testpage.sh

@@ -0,0 +1,43 @@
+#!/bin/bash
+# Deterministic test page for visual-parity captures. No prompt, no variance.
+E=$'\e'
+printf '\e[?25l\e[2J\e[H'
+printf '%s\n' 'ASCII: The quick brown fox jumps over the lazy dog 0123456789 {}[]()<>'
+printf '%s\n' 'Ligs: -> => == != >= <= :: |> <| www fi fl ffi'
+printf '%s\n' 'Sym: !"#$%&'"'"'()*+,-./:;<=>?@[\]^_`{|}~ gjpqy ÀÉÎÕÜ ñ ß'
+printf '\e[1mBold\e[0m \e[3mItalic\e[0m \e[1;3mBoldItalic\e[0m \e[2mFaint\e[0m \e[7mInverse\e[0m \e[9mStrike\e[0m \e[53mOverline\e[0m \e[8mhidden\e[0m|\n'
+printf 'UL: \e[4mSingle\e[0m \e[4:2mDouble\e[0m \e[4:3mCurly\e[0m \e[4:4mDotted\e[0m \e[4:5mDashed\e[0m \e[4:3;58;2;255;0;0mRedCurly\e[0m \e[4;9mUL+Strike\e[0m\n'
+printf '16: '
+for i in 0 1 2 3 4 5 6 7; do printf '\e[4%dm  \e[0m' $i; done; printf ' '
+for i in 0 1 2 3 4 5 6 7; do printf '\e[10%dm  \e[0m' $i; done; printf '\n'
+printf 'FG: '
+for i in 0 1 2 3 4 5 6 7; do printf '\e[3%dmAb\e[0m' $i; done; printf ' '
+for i in 0 1 2 3 4 5 6 7; do printf '\e[9%dmAb\e[0m' $i; done; printf '\n'
+printf '256: '
+for i in $(seq 16 87); do printf '\e[48;5;%dm \e[0m' $i; done; printf '\n     '
+for i in $(seq 88 159); do printf '\e[48;5;%dm \e[0m' $i; done; printf '\n     '
+for i in $(seq 160 231); do printf '\e[48;5;%dm \e[0m' $i; done; printf '\n     '
+for i in $(seq 232 255); do printf '\e[48;5;%dm  \e[0m' $i; done; printf '\n'
+printf 'RGB: '
+for i in $(seq 0 4 255); do printf '\e[48;2;%d;%d;%dm \e[0m' $i $((255-i)) 128; done; printf '\n'
+printf 'FGRGB: \e[38;2;255;128;0mOrange\e[0m \e[38;2;0;200;255;48;2;40;40;90mCyanOnBlue\e[0m \e[38;5;196mRed196\e[0m \e[38;5;21;48;5;226mBlueOnYellow\e[0m\n'
+printf 'Box: ┌─┬─┐ ╔═╦═╗ ╭─╮ ┏━┳━┓ │ │ │ ║ ║ ║ ├─┼─┤ ╠═╬═╣ ╰─╯ ┗━┻━┛ ╱╲╳ ┈┉┊┋ ╌╍╎╏\n'
+printf '     └─┴─┘ ╚═╩═╝ ┐┘└┌ ╒╓╕╖ ╞╟╡╢ ╤╥╧╨ ╪╫ ╴╵╶╷ ╸╹╺╻ ╼╽╾╿\n'
+printf 'Blk: ▀▁▂▃▄▅▆▇█▉▊▋▌▍▎▏▐░▒▓▔▕▖▗▘▙▚▛▜▝▞▟ ◢◣◤◥ ⬛\n'
+printf 'Brl: ⠀⠁⠂⠃⠄⠅⠆⠇⡀⡁⣀⣿⠿⣤⢕⡪ ⣾⣽⣻⢿⡿⣟⣯⣷\n'
+printf 'Pwr:               \n'
+printf '\e[30;44m \e[34;42m\e[30m main \e[32;40m\e[0m\n'
+printf 'Nerd:        \U000f0219   \n'
+printf 'CJK: 日本語のテキスト 中文字符 한국어 ｆｕｌｌ ＡＢＣ\n'
+printf 'Emoji: 😀 🎉 🚀 ❤️ 👍🏽 👨‍👩‍👧 🇩🇪 ✅ ⭐ ☀\n'
+printf 'Wide: |日|本|a|語|\n'
+printf 'Math: ∑ ∫ ∂ √ ∞ ≠ ≤ ≥ ← → ↑ ↓ ⇒ λ π Ω αβγ \n'
+printf 'Curs: '
+printf '\e[1 q\e[?25hX'
+printf '\e[?25l\e[0 q'
+printf '\n'
+printf '\e[7mReverse\e[27m \e[38;5;240;48;5;238mdim on dim\e[0m \e[31;41mred on red\e[0m\n'
+printf '\e[5;1H'
+printf '\e[24;1H'
+printf '\e[1 q\e[?25h'
+exec sleep 100000