Fix radius var fallback detection (#687)

Strip closing var() parentheses before resolving fallback radius tokens, preserving on-scale values and actionable ignore values.

AI-assisted change: implemented with Codex under @pbakaus direction.

cli/engine/design-system.mjs

@@ -995,7 +995,9 @@ function extractRadiusTokens(value) {
   return String(value || '')
     .replace(/\s*\/\s*/g, ' ')
     .split(/\s+/)
-    .map(token => token.trim())
+    // var() fallbacks leave the closing parenthesis on the final token. Strip
+    // it before length resolution so `8px)` is not treated as unitless 8rem.
+    .map(token => token.trim().replace(/\)+$/, ''))
     .filter(Boolean);
 }
 

tests/design-system.test.mjs

@@ -389,6 +389,25 @@ describe('checkSourceDesignSystem()', () => {
     );
   });
 
+  it('judges var() radius fallbacks without keeping the closing parenthesis', () => {
+    const designSystem = normalizeDesignSystem({
+      frontmatter: { rounded: { md: '8px' } },
+    });
+    const findings = checkSourceDesignSystem(`
+.good {
+  border-radius: var(--radius-md, 8px);
+}
+.bad {
+  border-radius: var(--radius-custom, 18px);
+}
+`, '/tmp/radius-fallbacks.css', { designSystem });
+
+    assert.deepEqual(
+      findings.map((item) => [item.antipattern, item.ignoreValue]),
+      [['design-system-radius', '18px']],
+    );
+  });
+
   it('strips CSS priority markers before checking font-family declarations', () => {
     const designSystem = sampleDesignSystem();
     const findings = checkSourceDesignSystem(`