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)

docs/rewrite/notes/visual-parity-VP.md

@@ -0,0 +1,62 @@
+# Visual parity vs original Ghostty 1.3.1 (agent VP)
+
+## Method (`scripts/visual-parity.sh`, `just visual-parity [scenario...]`)
+
+- A private headless `weston` (`--backend=headless --renderer=gl`, 1200x800, `desktop-shell`, no
+  panel) per run, on its own socket in `$XDG_RUNTIME_DIR=/tmp/vp-runtime-$UID` (socket paths must be
+  <108 bytes). The user's desktop is never touched. Mesa is forced for weston (`__EGL_VENDOR_LIBRARY_FILENAMES`
+  -> 50_mesa.json); the NVIDIA EGL crashes Xwayland's glamor. `XWAYLAND_NO_GLAMOR=1`.
+- `weston --debug` is what authorizes `weston-screenshooter` (without it: "Output capture error: unauthorized").
+- Both apps run with `env -i`, `GSETTINGS_BACKEND=memory`, no D-Bus, isolated `XDG_CONFIG_HOME` with the
+  same explicit config (JetBrains Mono 13, bg/fg, padding 8, `maximize = true` so the window geometry
+  is deterministic, `cursor-style-blink = false`, `title = Ghostty`, no shell integration) and
+  `LANG=C.UTF-8` (otherwise `printf '\U..'` in the test page prints literally).
+- Scenarios: `page` (block cursor), `page-bar` (bar cursor), `tabs` (new tab + split right + split down,
+  driven with `xdotool` through Xwayland since the original Ghostty has no input replay; Rust is driven
+  the same way so both apps are treated identically).
+  Test page: `scripts/visual-parity/testpage.sh` (ASCII, ligatures, bold/italic/faint/inverse/strike/
+  overline, all underline styles + colored, 16/256/truecolor, box drawing, blocks, braille, powerline,
+  nerd font icons, CJK, emoji, wide chars, math, cursor). Not covered: mouse selection (no input
+  injection into the original on Wayland; could be added through the Xwayland path).
+- `scripts/visual-parity/compare.py`: per-pixel max-channel delta, % differing, % > 8 / > 32 levels, max,
+  mean, red heat-map PNG; run on the full frame, the terminal area and the headerbar strip.
+  `scripts/visual-parity/zoom.py`: ref | rust | diff crops, upscaled.
+- Output (not committed): `target/visual-parity/<scenario>/{ref,rust,diff,diff-term,diff-chrome}.png`,
+  `target/visual-parity/stats.txt`. `RUST_BIN` defaults to `target/production/ghostty`.
+  Iteration build: `CARGO_TARGET_DIR=target/vp CARGO_PROFILE_PRODUCTION_LTO=off
+  CARGO_PROFILE_PRODUCTION_CODEGEN_UNITS=16 CARGO_PROFILE_PRODUCTION_INCREMENTAL=true cargo build --profile production`.
+
+## Results (% of pixels differing at all; max channel delta)
+
+| scenario | before | after |
+|---|---|---|
+| page (full frame) | 0.130% (1250 px, max 93) | 0.020% (196 px, max 86; 0.001% > 32 levels) |
+| page-bar | same as page | 0.020% |
+| tabs + splits | 44.6% (a whole pane blank, bug) | 0.000% (bit-identical) |
+| chrome (headerbar, tab bar) | 0.000% | 0.000% |
+
+Sprite sheets (`ghostty-font` tests, Zig reference PNGs): differing pixels 5.76% -> 1.20%, pixels off by
+more than 96 levels 74 -> 0 (test tolerance tightened from 4% to 0.1% big diffs).
+
+Font metrics, cell size, baseline, glyph rasterization, colors, cursor, underline positions, padding,
+GTK CSS/headerbar/tab bar were already pixel-identical; no change was needed there.
+
+## Fixes
+
+1. **Split rebuild blanked an unchanged pane** (`ghostty-core/src/surface/mod.rs`, `draw_frame`): the GTK
+   `render` handler called `renderer.draw_frame(false)`; Zig's `glRender` uses `drawFrame(true)`. With
+   `sync = false` and nothing dirty the renderer only re-presents the last target, which is cleared when
+   the GLArea is re-realized (nested split = tree rebuild = unrealize/realize of every surface whose size
+   did not change), so that pane stayed empty until something changed. Now `true` like Zig.
+2. **Sprite antialiasing** (`ghostty-font/src/sprite/canvas.rs`): replaced tiny-skia's coverage with a port of
+   z2d 0.10's default `multisample_4x` rasterizer (4x4 samples, scanline spans with edge x rounded at
+   sample row centers, non-zero filter, alpha = `coverage * 16 - 1`, full = 255, z2d's Bezier
+   flattening with tolerance 0.1). Stroke outlines still come from tiny-skia's stroker.
+
+## Remaining differences (page: 196 px, all <= 86 levels)
+
+- Curly underline: first/last pixels and peaks differ slightly. z2d builds its own stroke polygon (pen/joins)
+  for curves; tiny-skia's stroker offsets the Bezier differently.
+- Diagonal box glyphs (`U+2571..2573`) have a few edge pixels off by up to ~38 levels (same cause).
+- Porting z2d's stroker (`tess/stroke_plotter`, `Pen`) would remove the rest; judged not worth it
+  (0.02% of pixels, imperceptible).