refactor(audio-player): one source for the speed list (plan §5)

The rate list was duplicated — hardcoded in the templ x-for AND
implicitly in audio.ts — and phase 2's keyboard stepping would have
needed a third copy. SPEEDS is now the single exported const (the
agreed nine values, 0.5 through 3); the component exposes it as
'speeds' for the dropdown's x-for (templ cannot import TS), and
stepSpeed() walks the same array for the future audio-keys consumer.

normalizeSpeed() maps persisted values onto the list: garbage or
out-of-list localStorage entries fall back to 1x, and init() writes
the normalized value back so storage heals instead of staying
poisoned.

Pure logic unit-tested in audio.test.ts; live-verified with Playwright
on the KJV folder book: nine options render, pick/reload/persist and
the stale-value fallback all behave, button label and
audio.playbackRate agree in every case.

web/src/audio.ts

@@ -52,6 +52,28 @@ export function globalSecondsFor(
   return (track ? track.start : 0) + Math.max(0, localSeconds);
 }
 
+// Playback speeds — ONE source. The templ dropdown iterates it through
+// the component's `speeds` property, and keyboard stepping (audio-keys,
+// player plan §2) walks the same array through stepSpeed. Never hardcode
+// a rate list elsewhere.
+export const SPEEDS: readonly number[] = [0.5, 0.75, 1, 1.25, 1.5, 1.75, 2, 2.5, 3];
+
+// normalizeSpeed maps a persisted speed onto the SPEEDS list: missing,
+// malformed or out-of-list values (e.g. a stale localStorage entry from
+// before the list changed) fall back to 1×.
+export function normalizeSpeed(value: string | null): number {
+  const n = Number(value);
+  return SPEEDS.includes(n) ? n : 1;
+}
+
+// stepSpeed moves through SPEEDS by delta, clamping at both ends; an
+// unknown current anchors at 1× before stepping.
+export function stepSpeed(current: number, delta: 1 | -1): number {
+  const idx = SPEEDS.indexOf(current);
+  const base = idx === -1 ? SPEEDS.indexOf(1) : idx;
+  return SPEEDS[Math.min(SPEEDS.length - 1, Math.max(0, base + delta))];
+}
+
 const PROGRESS_INTERVAL_MS = 30_000;
 
 interface AudioConfig {
@@ -85,7 +107,10 @@ Alpine.data("audioReader", () => ({
   chaptersOpen: false,
   currentTime: 0,
   playing: false,
-  speed: Number(localStorage.getItem("audio_speed") || 1),
+  speed: normalizeSpeed(localStorage.getItem("audio_speed")),
+  // Exposed for the templ dropdown's x-for — templ cannot import the
+  // const directly; this property is the binding.
+  speeds: SPEEDS,
   skipSeconds: Number(localStorage.getItem("audio_skip") || 10),
   // Scrub guard: while dragging, timeupdate must not fight the handle
   scrubbing: false,
@@ -143,6 +168,12 @@ Alpine.data("audioReader", () => ({
   init() {
     if (this.initialized) return; // Alpine can re-init the body tree
     this.initialized = true;
+    // Heal a stale/out-of-list persisted speed so the storage matches
+    // what the player actually uses (normalizeSpeed already fixed the
+    // in-memory value at state creation).
+    if (localStorage.getItem("audio_speed") !== String(this.speed)) {
+      localStorage.setItem("audio_speed", String(this.speed));
+    }
     window.addEventListener("error", (e) => {
       document.title = "ERR: " + (e.message || "unknown");
     });
@@ -163,7 +194,7 @@ Alpine.data("audioReader", () => ({
     // src is set by restorePosition once the track manifest is known —
     // for multi-file books data-file-url (a directory or a zip root) is
     // NOT directly playable. The catch branch below falls back to it.
-    audio.playbackRate = Number(this.speed) || 1;
+    audio.playbackRate = this.speed;
     audio.preload = "auto";
     // Restored volume/mute from the settings strip's persisted state.
     audio.volume = Math.min(1, Math.max(0, Number(this.volume)));
@@ -303,7 +334,7 @@ Alpine.data("audioReader", () => ({
     } else {
       this.pendingSeek = Math.max(0, localSeconds);
       audio.setAttribute("src", track.url);
-      audio.playbackRate = Number(this.speed) || 1;
+      audio.playbackRate = this.speed;
       audio.load();
     }
     if (resume) audio.play().catch(() => showToast("Playback failed to start", "error"));