feat: expose stable journey intent signals

tests/unit/section-controller.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, it } from "vitest";
 import {
   activationLineDocumentY,
+  deriveSectionActivationSnapshot,
   deriveSectionState,
   isAtDocumentEnd,
   selectActiveSection,
@@ -109,6 +110,41 @@ describe("active section selection", () => {
     ).toBe("profile");
   });
 
+  it("derives responsive heading offsets from the same activation-line geometry", () => {
+    expect(deriveSectionActivationSnapshot({
+      headings: [
+        { id: "start", top: 100 },
+        { id: "profile", top: 420 },
+        { id: "experience", top: 1_120 },
+      ],
+      activeSectionId: "profile",
+      activationY: 500,
+      viewportHeight: 800,
+      layoutRevision: 7,
+    })).toEqual({
+      layoutRevision: 7,
+      activeHeadingOffsetVh: -0.1,
+      nextHeadingOffsetVh: 0.775,
+    });
+  });
+
+  it("returns no following-heading offset for the final active heading", () => {
+    expect(deriveSectionActivationSnapshot({
+      headings: [
+        { id: "start", top: 100 },
+        { id: "profile", top: 420 },
+      ],
+      activeSectionId: "profile",
+      activationY: 500,
+      viewportHeight: 800,
+      layoutRevision: 8,
+    })).toEqual({
+      layoutRevision: 8,
+      activeHeadingOffsetVh: -0.1,
+      nextHeadingOffsetVh: null,
+    });
+  });
+
   it("maps every canonical section to its chapter and arrival state", () => {
     expect([
       deriveSectionState("start"),