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/compare.py

@@ -0,0 +1,54 @@
+#!/usr/bin/env python3
+"""Pixel compare two PNG captures: stats + diff heatmap.
+
+usage: compare.py REF.png RUST.png DIFF.png [--crop x,y,w,h] [--name NAME]
+Prints one line: name, size, differing pixels (%), >8 / >32 levels, max delta, mean delta.
+"""
+import sys
+import numpy as np
+from PIL import Image
+
+
+def main():
+    a = sys.argv[1:]
+    crop = None
+    name = "all"
+    pos = []
+    i = 0
+    while i < len(a):
+        if a[i] == "--crop":
+            crop = tuple(int(v) for v in a[i + 1].split(","))
+            i += 2
+        elif a[i] == "--name":
+            name = a[i + 1]
+            i += 2
+        else:
+            pos.append(a[i])
+            i += 1
+    ref = Image.open(pos[0]).convert("RGB")
+    rs = Image.open(pos[1]).convert("RGB")
+    if crop:
+        x, y, w, h = crop
+        ref, rs = ref.crop((x, y, x + w, y + h)), rs.crop((x, y, x + w, y + h))
+    if ref.size != rs.size:
+        print(f"{name}: SIZE MISMATCH {ref.size} vs {rs.size}")
+        w, h = min(ref.size[0], rs.size[0]), min(ref.size[1], rs.size[1])
+        ref, rs = ref.crop((0, 0, w, h)), rs.crop((0, 0, w, h))
+    r = np.asarray(ref).astype(int)
+    s = np.asarray(rs).astype(int)
+    d = np.abs(r - s).max(axis=2)
+    n = d.size
+    diff = int((d > 0).sum())
+    print(
+        f"{name}: {ref.size[0]}x{ref.size[1]} differing={diff} ({100*diff/n:.3f}%) "
+        f">8={100*(d>8).sum()/n:.3f}% >32={100*(d>32).sum()/n:.3f}% max={d.max()} mean={d.mean():.4f}"
+    )
+    # heatmap: dimmed reference with differences in red (intensity ~ delta)
+    heat = (r * 0.25).astype(np.uint8)
+    m = np.clip(d * 4, 0, 255).astype(np.uint8)
+    heat[..., 0] = np.maximum(heat[..., 0], m)
+    heat[..., 1] = np.where(m > 0, heat[..., 1] // 2, heat[..., 1])
+    Image.fromarray(heat).save(pos[2])
+
+
+main()