fix(lint): stop font_family_without_font_face flagging system-ui stacks and var() (#1796)

The font_family_without_font_face rule raised a blocking error on two
legitimate, very common cases:

1. The `-apple-system, BlinkMacSystemFont` system-ui stack. These two
   tokens are the cross-browser incantation for the platform UI font
   (synonyms of `system-ui`); they name no font file, so demanding an
   @font-face for them is wrong. They appear in almost every CSS reset.

2. `font-family: var(--x)` indirection. The shared family extractor took
   the literal `var(--heading)` as a font name. The linter cannot
   statically resolve a custom property, so it must not flag it.

Fix at the root: add the two system-ui synonyms to GENERIC_FAMILIES, and
skip any parenthesised (function) token in the shared family extractor so
both this rule and system_font_will_alias stop misreading var(). A real
undeclared font sitting in the same stack is still flagged.

packages/lint/src/rules/fonts.test.ts

@@ -291,5 +291,39 @@ describe("font rules", () => {
       const findings = await findByCode(html, "font_family_without_font_face");
       expect(findings).toHaveLength(0);
     });
+
+    it("does not flag the -apple-system / BlinkMacSystemFont system-ui stack", async () => {
+      const html = `<div data-composition-id="test" data-width="1920" data-height="1080">
+        <style>body { font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif; }</style>
+      </div>`;
+      const findings = await findByCode(html, "font_family_without_font_face");
+      expect(findings).toHaveLength(0);
+    });
+
+    it("does not flag a var() font-family indirection it cannot resolve", async () => {
+      const html = `<div data-composition-id="test" data-width="1920" data-height="1080">
+        <style>:root { --heading: 'Inter'; } h1 { font-family: var(--heading); }</style>
+      </div>`;
+      const findings = await findByCode(html, "font_family_without_font_face");
+      expect(findings).toHaveLength(0);
+    });
+
+    it("does not flag a var() with a quoted fallback font", async () => {
+      const html = `<div data-composition-id="test" data-width="1920" data-height="1080">
+        <style>h1 { font-family: var(--heading, 'Geist'), sans-serif; }</style>
+      </div>`;
+      const findings = await findByCode(html, "font_family_without_font_face");
+      expect(findings).toHaveLength(0);
+    });
+
+    it("still flags a real undeclared font sitting next to a system stack", async () => {
+      const html = `<div data-composition-id="test" data-width="1920" data-height="1080">
+        <style>body { font-family: 'Aeonik', -apple-system, BlinkMacSystemFont, sans-serif; }</style>
+      </div>`;
+      const findings = await findByCode(html, "font_family_without_font_face");
+      expect(findings).toHaveLength(1);
+      expect(findings[0]!.message).toContain("aeonik");
+      expect(findings[0]!.message).not.toContain("apple-system");
+    });
   });
 });

packages/lint/src/rules/fonts.ts

@@ -56,6 +56,22 @@ function extractFontFaceFamilies(styles: Array<{ content: string }>): Set<string
   return families;
 }
 
+// Normalize one comma-separated font-family entry to a lowercase family name,
+// or null if it carries no resolvable name. `var(--heading)` (or any function
+// token) is an indirection the linter cannot statically resolve, so the literal
+// `var(...)` is not a font name and flagging it is a false positive. Comma-split
+// fallbacks like `var(--x, 'Inter')` also leave a dangling `)` on the fallback
+// part, so skip anything bearing parentheses.
+function normalizeUsedFontName(part: string): string | null {
+  const name = part
+    .trim()
+    .replace(/^['"]|['"]$/g, "")
+    .trim()
+    .toLowerCase();
+  if (!name || name.includes("(") || name.includes(")")) return null;
+  return name;
+}
+
 function extractUsedFontFamilies(styles: Array<{ content: string }>): string[] {
   const used: string[] = [];
   const seen = new Set<string>();
@@ -64,13 +80,8 @@ function extractUsedFontFamilies(styles: Array<{ content: string }>): string[] {
     const withoutFontFace = stripCssComments(style.content).replace(/@font-face\s*\{[^}]*\}/gi, "");
     let match: RegExpExecArray | null;
     while ((match = propRe.exec(withoutFontFace)) !== null) {
-      const stack = match[1]!;
-      for (const part of stack.split(",")) {
-        const name = part
-          .trim()
-          .replace(/^['"]|['"]$/g, "")
-          .trim()
-          .toLowerCase();
+      for (const part of match[1]!.split(",")) {
+        const name = normalizeUsedFontName(part);
         if (name && !GENERIC_FAMILIES.has(name) && !seen.has(name)) {
           seen.add(name);
           used.push(name);