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>

modules/home/swaync.nix

@@ -29,11 +29,14 @@ in
       };
 
       style = ''
-        /* Palette baked from theme-state.json */
+        /* Palette baked from theme-state.json. Only roles guaranteed to
+           contrast @base in EVERY palette are used: subtext/surface mean
+           "on-surface" in some light themes (summer-day: subtext==base,
+           surface==text — body text was invisible on hardware, item 25).
+           Chips/hovers are alpha(@text) tints — contrast by construction.
+           tools/check-theme-contrast.py guards the hex-on-hex pairings. */
         @define-color base ${c.base};
-        @define-color surface ${c.surface};
         @define-color text ${c.text};
-        @define-color subtext ${c.subtext};
         @define-color accent ${c.accent};
         @define-color bad ${c.bad};
 
@@ -50,7 +53,7 @@ in
         }
 
         .notification-content .body {
-          color: @subtext;
+          color: @text;
         }
 
         .notification.critical {
@@ -66,7 +69,7 @@ in
 
         .control-center .notification-row:focus,
         .control-center .notification-row:hover {
-          background: alpha(@surface, 0.6);
+          background: alpha(@text, 0.1);
           border-radius: ${r}px;
         }
 
@@ -76,7 +79,7 @@ in
         }
 
         .widget-title > button {
-          background: @surface;
+          background: alpha(@text, 0.1);
           color: @text;
           border: none;
           border-radius: ${r}px;