fix: a chapter's first mile-marker post no longer arrives late

tests/unit/milestones.test.ts

@@ -9,6 +9,8 @@ import {
   MILESTONE_ASSIGN_Z,
   MILESTONE_FAR_Z,
   MILESTONE_FLASH_MAX_RATE,
+  MILESTONE_LATE_ASSIGN_Z,
+
   MILESTONE_NEAR_Z,
   MILESTONE_PASS_Z,
   MILESTONE_SIDE,
@@ -200,7 +202,53 @@ describe('mile markers on the road', () => {
     expect(seen.filter((pass) => pass.reverse).map((pass) => pass.entry)).toEqual([2, 1, 0]);
   });
 
-  it('only hands an entry to a post still far up the road, in the haze', () => {
+  it('does not keep the first entry waiting when the reader starts scrolling at the heading', () => {
+    for (const [section, rate, speed] of [
+      ['profile', 0.03, 18],
+      ['profile', 0.05, 18],
+      ['experience', 0.03, 18],
+      ['experience', 0.05, 25],
+    ] as const) {
+      const count = MILESTONE_COUNTS[section];
+      for (const lead of [0, 0.4, 1.7, 2.9]) {
+        const road = createMilestoneRoad(true);
+        // Cruising through the chapter before, then straight into reading from the heading.
+        const state = drive(road, 3 + lead, () => ({ section: 'start', p: null, speed }));
+        const seen: Array<{ entry: number; p: number }> = [];
+        let last: readonly number[] = road.posts().map((post) => post.z);
+        drive(road, 1 / rate, (t) => ({ section, p: Math.min(1, t * rate), speed }), state, (posts, _t, p) => {
+          posts.forEach((post, k) => {
+            if (post.entry >= 0 && post.presence > 0 && last[k]! < MILESTONE_PASS_Z && post.z >= MILESTONE_PASS_Z) {
+              seen.push({ entry: post.entry, p: p! });
+            }
+          });
+          last = posts.map((post) => post.z);
+        });
+        expect(seen[0]?.entry).toBe(0);
+        // Within a post's spacing of the line (the road brings one every 60 m); it used to be up to two entries late.
+        expect(Math.abs(seen[0]!.p - passPoint(0, count))).toBeLessThan(0.4 / count);
+      }
+    }
+  });
+
+  it('sends the opening entry by soon when the reader stays at the heading', () => {
+    for (const lead of [0, 0.4, 1.7, 2.9]) {
+      const road = createMilestoneRoad(true);
+      const state = drive(road, 3 + lead, () => ({ section: 'start', p: null, speed: 18 }));
+      let first = Number.POSITIVE_INFINITY;
+      let last: readonly number[] = road.posts().map((post) => post.z);
+      drive(road, 12, () => ({ section: 'profile', p: 0, speed: 18 }), state, (posts, t) => {
+        posts.forEach((post, k) => {
+          if (post.entry === 0 && post.presence > 0 && last[k]! < MILESTONE_PASS_Z && post.z >= MILESTONE_PASS_Z) first = Math.min(first, t);
+        });
+        last = posts.map((post) => post.z);
+      });
+      // A post already on the road, not one from the haze (~8.5 s at this speed).
+      expect(first).toBeLessThan(6.5);
+    }
+  });
+
+  it('only hands an entry to a post still far up the road, or to a dark one as a chapter begins', () => {
     const road = createMilestoneRoad(true);
     const born = new Map<number, number>();
     drive(road, 30, (t) => ({ section: 'experience', p: Math.min(1, t * 0.03), speed: 30 }), { distance: 0 }, (posts) => {
@@ -209,7 +257,10 @@ describe('mile markers on the road', () => {
       });
     });
     expect(born.size).toBeGreaterThan(0);
-    for (const z of born.values()) expect(z).toBeLessThan(MILESTONE_ASSIGN_Z);
+    // The opening entry may go to a nearer post that was still dark; every later one comes from the haze.
+    const zs = [...born.values()];
+    expect(zs[0]!).toBeLessThan(MILESTONE_LATE_ASSIGN_Z);
+    for (const z of zs.slice(1)) expect(z).toBeLessThan(MILESTONE_ASSIGN_Z);
   });
 });