feat(audio-player): progress bar overhaul — ticks, buffer, wheel, tooltip, precision rail (plan §1)

The scrubber becomes a layered bar: the native <input type=range> stays
on top for a11y (focus, arrow keys, aria — its track goes transparent
via static page CSS) while the layers beneath render the state:

- Chapter ticks: density-decimated (denseTicks keeps only marks >= 4px
  apart) so an 1189-chapter 86-hour book gets ~100 readable marks
  instead of a solid mass; the current chapter's tick is tinted.
- Buffered span: the ACTIVE track's buffered-ahead end mapped into the
  global bar (buffers are per-track and never span the whole bar).
  bufferedAheadEnd reads the range containing the playhead — a huge
  m4b's moov index at the file tail is its own buffered range and must
  not read as 'the whole book is buffered'. Rides progress + the
  events that demonstrably fire for Range-served files that go
  network-idle while paused (timeupdate/canplay/loadedmetadata/playing).
- Wheel over the bar: ±5s nudge, Shift = ±60s.
- Hover tooltip: time + chapter title under the pointer.
- Precision rail: while dragging, a ±5-minute window floats below the
  bar (fixed anchor; release and re-grab to travel far) and the drag
  maps at minutes-per-pixel instead of hours-per-pixel — the whole
  point for 80h+ books. Click snaps to a chapter line within ~5s
  (snapToChapter).

Pointer handling is ours end-to-end: pointerdown maps hours-per-pixel
+ snaps, captured moves remap through the rail, release seeks. The
native drag default is prevented so the two mappings never fight;
keyboard arrows keep their native step and seek via change.

Time labels get reserved width — the bar no longer resizes when the
elapsed label grows from 0:00 to 43:07 mid-drag (caught by the rail
drag check measuring a 25% mapping drift).

Pure math in audio-scrubber.ts: pointer mapping, chapter lookup, snap,
tick density, per-track buffer→global span, rail window/mapping/ticks.
Unit: 16 new tests including both book shapes (86h/1189ch and
26h/34ch). Live: 23 Playwright checks on KJV (folder) and Gardens of
the Moon (single-file m4b); zip books share the folder manifest shape
(no zip fixture exists in the dev library). Probe user deleted.

web/src/audio-scrubber.test.ts

@@ -0,0 +1,208 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  bufferedAheadEnd,
+  bufferedSpanPct,
+  chapterIndexFor,
+  denseTicks,
+  railSecondsAtPointer,
+  railTicks,
+  railWindow,
+  RAIL_SPAN,
+  secondsAtPointer,
+  snapToChapter,
+} from "./audio-scrubber";
+
+// An 86-hour book (KJV-shaped: 1189 even chapters) and a 26-hour
+// single-file book (Gardens-shaped: 34 chapters).
+const duration86h = 310_320;
+const chapters1189 = Array.from({ length: 1189 }, (_, i) => ({ start: (i * duration86h) / 1189 }));
+const duration26h = 93_807;
+const chapters34 = Array.from({ length: 34 }, (_, i) => ({
+  start: (i * duration26h) / 34,
+  title: `Chapter ${i + 1}`,
+}));
+
+const rect = (width: number): DOMRect =>
+  ({
+    left: 100,
+    right: 100 + width,
+    width,
+    top: 0,
+    bottom: 10,
+    height: 10,
+    x: 100,
+    y: 0,
+    toJSON: () => ({}),
+  }) as DOMRect;
+
+describe("secondsAtPointer", () => {
+  it("maps the bar linearly and clamps outside it", () => {
+    const r = rect(800);
+    expect(secondsAtPointer(100, r, duration86h)).toBe(0);
+    expect(secondsAtPointer(500, r, duration86h)).toBeCloseTo(duration86h / 2, 5);
+    expect(secondsAtPointer(900, r, duration86h)).toBe(duration86h);
+    expect(secondsAtPointer(50, r, duration86h)).toBe(0);
+    expect(secondsAtPointer(1200, r, duration86h)).toBe(duration86h);
+  });
+
+  it("is safe with degenerate rects and durations", () => {
+    expect(secondsAtPointer(150, rect(0), duration86h)).toBe(0);
+    expect(secondsAtPointer(150, rect(800), 0)).toBe(0);
+  });
+});
+
+describe("chapterIndexFor", () => {
+  it("finds the containing chapter in both shapes", () => {
+    expect(chapterIndexFor(chapters1189, 0)).toBe(0);
+    expect(chapterIndexFor(chapters1189, chapters1189[500].start + 1)).toBe(500);
+    expect(chapterIndexFor(chapters34, chapters34[33].start + 60)).toBe(33);
+    expect(chapterIndexFor(chapters34, duration26h + 5000)).toBe(33);
+  });
+
+  it("returns -1 before the first chapter and for none at all", () => {
+    expect(chapterIndexFor(chapters1189, -1)).toBe(-1);
+    expect(chapterIndexFor([], 100)).toBe(-1);
+  });
+});
+
+describe("snapToChapter", () => {
+  it("snaps within the threshold and not beyond it", () => {
+    const start = chapters1189[10].start;
+    expect(snapToChapter(start + 3, chapters1189, 5)).toEqual({ seconds: start, snapped: true });
+    expect(snapToChapter(start + 4.9, chapters1189, 5)).toEqual({ seconds: start, snapped: true });
+    expect(snapToChapter(start + 5.1, chapters1189, 5).snapped).toBe(false);
+    expect(snapToChapter(start - 5, chapters1189, 5)).toEqual({ seconds: start, snapped: true });
+  });
+
+  it("picks the NEAREST line when two are in range", () => {
+    const a = chapters34[5].start;
+    const b = chapters34[6].start;
+    const mid = (a + b) / 2;
+    // Chapter gap (~2759s) means both lines sit ~1380s from the
+    // midpoint — the threshold must reach past it.
+    const result = snapToChapter(mid + 1, chapters34, 1500);
+    expect(result.snapped).toBe(true);
+    expect(result.seconds).toBe(b);
+  });
+
+  it("leaves the position alone with no chapters", () => {
+    expect(snapToChapter(42, [], 5)).toEqual({ seconds: 42, snapped: false });
+  });
+});
+
+describe("denseTicks", () => {
+  it("decimates an 1189-chapter book to a readable number of marks", () => {
+    const ticks = denseTicks(chapters1189, duration86h, 800, 4);
+    expect(ticks.length).toBeGreaterThan(50);
+    expect(ticks.length).toBeLessThan(300);
+    expect(ticks[0]).toBe(0);
+    // kept marks are strictly increasing and spaced by the gap rule
+    for (let i = 1; i < ticks.length; i += 1) {
+      const gapPx =
+        ((chapters1189[ticks[i]].start - chapters1189[ticks[i - 1]].start) / duration86h) * 800;
+      expect(gapPx).toBeGreaterThanOrEqual(4);
+    }
+  });
+
+  it("keeps every chapter for a sparse 34-chapter book", () => {
+    const ticks = denseTicks(chapters34, duration26h, 800, 4);
+    expect(ticks).toHaveLength(34);
+  });
+
+  it("handles degenerate inputs", () => {
+    expect(denseTicks([], duration86h, 800)).toEqual([]);
+    expect(denseTicks(chapters34, 0, 800)).toEqual([]);
+    expect(denseTicks(chapters34, duration26h, 0)).toEqual([]);
+  });
+});
+
+describe("bufferedAheadEnd", () => {
+  it("returns the end of the range containing the playhead", () => {
+    const ranges = [
+      { start: 0, end: 2 },
+      { start: 92_719, end: 93_808 },
+    ];
+    // A huge m4b's moov index at the tail is its own range — it must
+    // NOT read as "the whole book is buffered".
+    expect(bufferedAheadEnd(ranges, 0)).toBe(2);
+    expect(bufferedAheadEnd(ranges, 1.5)).toBe(2);
+    expect(bufferedAheadEnd(ranges, 92_800)).toBe(93_808);
+  });
+
+  it("walks into the next range when the playhead sits at a boundary", () => {
+    expect(
+      bufferedAheadEnd(
+        [
+          { start: 0, end: 10 },
+          { start: 10, end: 30 },
+        ],
+        10,
+      ),
+    ).toBe(30);
+  });
+
+  it("returns null when the playhead is in a gap or ranges are empty", () => {
+    expect(bufferedAheadEnd([{ start: 50, end: 60 }], 10)).toBeNull();
+    expect(bufferedAheadEnd([], 10)).toBeNull();
+  });
+});
+
+describe("bufferedSpanPct", () => {
+  it("maps only the active track's segment on a multi-track book", () => {
+    // Track 8 of the KJV shape spans ~2088s..2344s of 310320s.
+    const track = { start: 2088, end: 2344 };
+    const span = bufferedSpanPct(track, 2344, duration86h);
+    expect(span).not.toBeNull();
+    expect(span!.left).toBeCloseTo((2088 / duration86h) * 100, 5);
+    expect(span!.width).toBeCloseTo(((2344 - 2088) / duration86h) * 100, 5);
+    // Element buffer beyond the track end is clamped to the track.
+    const clamped = bufferedSpanPct(track, 99999, duration86h);
+    expect(clamped!.width).toBeCloseTo(((2344 - 2088) / duration86h) * 100, 5);
+  });
+
+  it("spans the whole bar for a single-file book (start 0, end unknown)", () => {
+    const span = bufferedSpanPct({ start: 0, end: 0 }, 40_000, duration26h);
+    expect(span!.left).toBe(0);
+    expect(span!.width).toBeCloseTo((40_000 / duration26h) * 100, 5);
+  });
+
+  it("returns null with no track or no buffer", () => {
+    expect(bufferedSpanPct(undefined, 100, duration86h)).toBeNull();
+    expect(bufferedSpanPct({ start: 0, end: 0 }, null, duration86h)).toBeNull();
+    expect(bufferedSpanPct({ start: 0, end: 0 }, 0, duration86h)).toBeNull();
+  });
+});
+
+describe("railWindow / railSecondsAtPointer / railTicks", () => {
+  it("centers a ±5min window and clamps at the book edges", () => {
+    const mid = railWindow(duration26h / 2, duration26h);
+    expect(mid.from).toBeCloseTo(duration26h / 2 - RAIL_SPAN / 2, 5);
+    expect(mid.to).toBeCloseTo(duration26h / 2 + RAIL_SPAN / 2, 5);
+    const atStart = railWindow(0, duration26h);
+    expect(atStart.from).toBe(0);
+    expect(atStart.to).toBe(RAIL_SPAN);
+    const atEnd = railWindow(duration26h, duration26h);
+    expect(atEnd.to).toBe(duration26h);
+    expect(atEnd.to - atEnd.from).toBe(RAIL_SPAN);
+    // A book shorter than the window clamps to the whole book.
+    expect(railWindow(60, 120)).toEqual({ from: 0, to: 120 });
+  });
+
+  it("maps pointer X at minutes-per-pixel inside the window", () => {
+    const r = rect(600);
+    const window = { from: 3000, to: 3600 };
+    expect(railSecondsAtPointer(100, r, window)).toBe(3000);
+    expect(railSecondsAtPointer(400, r, window)).toBe(3300);
+    expect(railSecondsAtPointer(700, r, window)).toBe(3600);
+  });
+
+  it("lists the chapter starts inside the window with titles", () => {
+    // from is 1s past chapter 4's start, so chapter 4 is outside.
+    const ticks = railTicks(
+      { from: chapters34[3].start + 1, to: chapters34[6].start + 1 },
+      chapters34,
+    );
+    expect(ticks.map((t) => t.title)).toEqual(["Chapter 5", "Chapter 6", "Chapter 7"]);
+  });
+});