Accessibility Design Token Enforcer setup

Implement the WCAG relative luminance formula from scratch in pure Dart — it is a well-defined algorithm (linearize sRGB values, then L = 0.2126R + 0.7152G + 0.0722B) and has no dependency. Contrast ratio = (L1 + 0.05) / (L2 + 0.05) where L1 is the lighter luminance. Guard ALL assertion method bodies with `if (!kDebugMode) return;` as the first line — this is more reliable than using assert() which can be compiled away depending on build configuration. For the AccessibilityTokenEnforcerWidget, use LayoutBuilder to get the rendered size of the child and compare against minTouchTarget — trigger the assertion in the layout phase, not paint phase, so the error is caught early.

To avoid false positives on decorative non-interactive widgets, require consumers to explicitly opt in by wrapping interactive widgets, rather than globally wrapping everything. The ContrastSafeColorPalette should be co-located with the existing design token theme file — add it as a static extension or a nested class, not a separate file, to make the relationship clear. Keep the token constants aligned with the AccessibilityTokens used in task-011 tests — these two tasks should reference the same source constants. Once this component exists, the CertificationStatusScreen audit (task-011) should import and use assertContrastRatio() in its tests.

Unit tests for contrast math: write golden-value tests for assertContrastRatio() using known WCAG examples — e.g., pure black on pure white = 21:1 (passes), mid-gray #767676 on white = 4.48:1 (barely passes AA normal text), light gray #AAAAAA on white = 2.32:1 (fails). Assert these exact values to lock in the luminance formula. Unit tests for assertion behavior: in debug mode, assert that assertTouchTarget(Size(40, 40)) throws a FlutterError; assert that assertTouchTarget(Size(48, 48)) does not throw. In release mode simulation (override kDebugMode with a test flag or use conditional compilation), assert that the same assertTouchTarget(Size(40, 40)) call does not throw.

Integration tests: place AccessibilityTokenEnforcerWidget around a button with a known-small touch target in a widget test and verify the FlutterError is thrown. ContrastSafeColorPalette tests: assert every named color pair in the palette passes its corresponding contrast ratio assertion — these tests serve as a regression guard against theme changes breaking contrast compliance. Run these tests in CI.