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

tests/unit/section-controller.test.ts

@@ -20,14 +20,14 @@ const headings: readonly SectionHeadingGeometry[] = [
 ];
 
 describe("active section selection", () => {
-  it("places the activation line at a literal 35 percent of the visual viewport", () => {
+  it("places the activation line at a literal 60 percent of the visual viewport", () => {
     expect(
       activationLineDocumentY({
         scrollY: 200,
         visualViewportOffsetTop: 25,
         visualViewportHeight: 1000,
       }),
-    ).toBe(575);
+    ).toBe(825);
   });
 
   it("forces document end within exactly two CSS pixels using current geometry", () => {
@@ -60,7 +60,7 @@ describe("active section selection", () => {
     ).toBe("start");
   });
 
-  it("selects the last heading at or above the literal 35 percent activation line", () => {
+  it("selects the last heading at or above the activation line", () => {
     expect(
       selectActiveSection({
         headings,