feat: light a chapter when its heading crosses 60% of the viewport

tests/browser/navigation.spec.ts

@@ -73,9 +73,9 @@ test("passive scrolling replaces a stable fragment without stealing focus", asyn
     if (heading === null) throw new Error("Missing experience heading");
     const headingDocumentTop =
       heading.getBoundingClientRect().top + window.scrollY;
-    // Place the heading just beyond the 35% activation line. Scroll offsets
+    // Place the heading just beyond the 60% activation line. Scroll offsets
     // are integer-rounded in Chromium, so exact boundary equality is brittle.
-    window.scrollTo(0, headingDocumentTop - window.innerHeight * 0.34);
+    window.scrollTo(0, headingDocumentTop - window.innerHeight * 0.59);
   });
 
   await expect(page).toHaveURL(`${distUrl}/#experience`, { timeout: 2000 });