fix(swaync): palette-safe pairings — text was invisible on summer-day (item 25)

Palette roles aren't uniform: summer-day and flexoki-light use subtext
as text-ON-surface (== base) and surface as a dark chip (== text), so
swaync's @subtext-on-@base body text and @surface/@text buttons were
self-colored (caught on the Latitude).

- body/summary: @text on @base — the one text pairing that passes in
  every palette (survey: worst WCAG ratio 5.18 across all 21 themes;
  subtext/base bottomed at 1.00 twice, muted/base at 1.27).
- hover rows + widget-title buttons: alpha(@text, 0.1) tints —
  contrast by construction; subtext/surface defines dropped.
- NEW checks.theme-contrast (tools/check-theme-contrast.py): asserts
  the hex-on-hex text pairings across themes/*.json, cheap (no VM).

The audit also found the generated waybar/rofi CSS shares the bug
class, live on flexoki-light (no whole-swap) → BACKLOG item 27.

Verified: V0; V1 (checks.theme-contrast builds green, 21 themes;
rendered services.swaync.style has zero @subtext/@surface). Honesty:
NOT visually verified — V3 queued (notify-send on summer-day).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/check-theme-contrast.py

@@ -0,0 +1,69 @@
+#!/usr/bin/env python3
+"""Guard against invisible-text palettes (BACKLOG item 25).
+
+Asserts, for every themes/*.json, the WCAG contrast ratio of each
+hex-on-hex fg/bg pairing the generated swaync CSS uses. Pairings built
+from alpha(@text) tints over @base are contrast-by-construction and
+need no assert. Extend PAIRINGS as more generated surfaces are fixed to
+palette-safe roles (item 27 tracks waybar/rofi, which still use
+subtext/surface and would fail here today).
+
+Wired as `checks.theme-contrast` in flake.nix; also runnable directly:
+    python3 tools/check-theme-contrast.py themes/
+"""
+import json
+import sys
+from pathlib import Path
+
+# (fg role, bg role, minimum ratio, consumer). Text pairings only —
+# borders (@accent/@bad on @base in swaync) are decorative and not held
+# to a text ratio (miasma's bad-on-base is 2.3, fine for a border).
+PAIRINGS = [
+    ("text", "base",   4.5, "swaync body/summary/titles on the toast"),
+    ("base", "accent", 3.0, "swaync widget-title button:hover"),
+]
+
+
+def channel(c: int) -> float:
+    c /= 255
+    return c / 12.92 if c <= 0.03928 else ((c + 0.055) / 1.055) ** 2.4
+
+
+def luminance(hexstr: str) -> float:
+    hexstr = hexstr.lstrip("#")
+    r, g, b = (int(hexstr[i:i + 2], 16) for i in (0, 2, 4))
+    return 0.2126 * channel(r) + 0.7152 * channel(g) + 0.0722 * channel(b)
+
+
+def ratio(fg: str, bg: str) -> float:
+    hi, lo = sorted((luminance(fg), luminance(bg)), reverse=True)
+    return (hi + 0.05) / (lo + 0.05)
+
+
+def main() -> int:
+    themes_dir = Path(sys.argv[1] if len(sys.argv) > 1 else "themes")
+    failures = []
+    themes = sorted(themes_dir.glob("*.json"))
+    if not themes:
+        print(f"no theme JSONs found under {themes_dir}", file=sys.stderr)
+        return 1
+    for path in themes:
+        colors = json.loads(path.read_text())["colors"]
+        for fg, bg, minimum, consumer in PAIRINGS:
+            r = ratio(colors[fg], colors[bg])
+            if r < minimum:
+                failures.append(
+                    f"{path.stem}: {fg} on {bg} = {r:.2f} < {minimum}"
+                    f" ({consumer}; {colors[fg]} on {colors[bg]})"
+                )
+    if failures:
+        print("theme contrast failures:", file=sys.stderr)
+        for f in failures:
+            print(f"  {f}", file=sys.stderr)
+        return 1
+    print(f"{len(themes)} themes x {len(PAIRINGS)} pairings: all pass")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())