Podcast voices: natural provider TTS, and system-voice hygiene

Two reports, one root: the platform's speech engine.

"Loud cracking at the end of every sentence" is the browser/OS TTS popping
between its internal sentence chunks (an idle cancel makes it worse). Hygiene
for that path: never cancel when nothing is pending, normalise the spoken text
(control characters, ellipses, dashes), and a 140ms beat between sections so
speech never runs into the engine's tail.

The real fix is a second voice source that does not use the platform engine:
- lib/providers carries TTS profiles (OpenAI gpt-4o-mini-tts, Groq
  playai-tts, any OpenAI-compatible /audio/speech behind the custom provider),
  male voices first.
- /api/tts proxies them with the same guards and the key still in headers.
- the picker gains System / Natural, with the provider's voice list.
- lib/podcast-audio plays one clip per section through a hidden <audio>
  element, measures the clip's real duration and gives each word a slice of it
  proportional to its spoken length - the highlight follows the audio exactly,
  and pause/seek/chapters become real audio operations. Clips cache per
  session, the next section generates while the current plays, and a voice
  change disposes the element and its blobs. Speed applies live (setRate)
  without dropping the cache.

podcast-test stubs /api/tts with a real silent WAV and pins the path: the
voice list, the request body, exactly one playing element, and the highlight
riding currentTime. 43 checks. tests/lib.mjs launches Chromium with the
autoplay policy relaxed (a real device has the listener's gesture).
Version 0.7.6, SW v98.

lib/podcast-speech.ts

@@ -21,6 +21,16 @@ import type { PodcastSection } from "./schemas";
 
 export type PodcastVoice = { section: number; word: number };
 
+/** Tidy text for the speech engine (see the note where it is used). */
+export function normaliseSpoken(raw: string): string {
+  return raw
+    .replace(/[\u0000-\u001f\u007f-\u009f\u200b-\u200f\ufeff]/g, " ")
+    .replace(/…/g, ", ")
+    .replace(/\s*—\s*/g, " — ")
+    .replace(/\s+/g, " ")
+    .trim();
+}
+
 export type PodcastSpeechEvents = {
   sections: PodcastSection[];
   locale: string;
@@ -129,10 +139,16 @@ export function createPodcastSpeech(events: PodcastSpeechEvents): PodcastSpeech
     position = { section: target.section, word };
 
     const from = sectionTokens[word].start;
-    const text = section.text.slice(from);
+    /* Spoken text hygiene: engines click on control characters, stray
+       double spaces and unicode ellipses, and some pause oddly around them. */
+    const text = normaliseSpoken(section.text.slice(from));
     const current = ++generation;
     try {
-      window.speechSynthesis.cancel();
+      /* Only cancel when something is actually pending: an idle cancel makes
+         several engines pop. */
+      if (window.speechSynthesis.speaking || window.speechSynthesis.pending) {
+        window.speechSynthesis.cancel();
+      }
       const utterance = new SpeechSynthesisUtterance(text);
       utterance.lang = events.locale;
       utterance.rate = events.rate;
@@ -200,7 +216,12 @@ export function createPodcastSpeech(events: PodcastSpeechEvents): PodcastSpeech
           events.onEnd();
           return;
         }
-        speakFrom({ section: target.section + 1, word: 0 });
+        /* A short beat between parts: speaking into the engine's tail is
+           what makes the boundary pop on several platforms. */
+        window.setTimeout(() => {
+          if (generation !== current) return;
+          speakFrom({ section: target.section + 1, word: 0 });
+        }, 140);
       };
 
       utterance.onerror = (event) => {