audit alert

docs/CHECKLIST.md

@@ -14,7 +14,6 @@ is the first downstream consumer; its screens drive the order.
 Platform metric adoption continues one component at a time; each stop goes
 through the review gate and a gallery pass.
 
-- [ ] **Alert platform chrome** — overlay border gate.
 - [ ] **Combobox platform chrome** — popup border behind
       `Theme::borders.overlays`; the trigger follows the input stop's field
       treatment.
@@ -127,6 +126,10 @@ Gaps surfaced by the signed comparison; ordered by signed's usage.
       it flips above the parent when it would overflow the window bottom
       (the panel records its own height after the first frame); unanchored
       tooltips fall back to the cursor-offset wrapper
+- [x] Alert platform chrome — the banner border behind
+      `Theme::borders.overlays` (and dark mode), taking the surface color
+      where the platform paints none; it rides the popover elevation, so
+      dark mode's no-shadow rule applies to it too
 - [x] Button, Checkbox, Switch, Radio + RadioGroup, Toggle, Link, Slider,
       Progress, Avatar (image + seeded pixel-art fallback)
 - [x] Spinner — painted ring + rotating-icon variant, indeterminate and