Podcast speech: the highlight clock learns the engine's pace

"The tts dispatcher thing and the text highlight is completely out of sync on
desktop." Firefox + speech-dispatcher (like iOS) reports no onboundary events,
so the highlight rode a pure estimate - and a 130-word section drifts seconds
away from an engine that speaks at its own pace.

- On platforms without boundaries the system engine now speaks one sentence
  at a time, and every finished utterance re-anchors the clock: measured
  duration vs estimate, smoothed (half old, half new, clamped to 0.25-4x).
  Drift cannot accumulate past a sentence.
- The per-word estimate is punctuation-aware (full stop ~260ms, comma ~130ms).
- Once a boundary does arrive (Chrome), the rest of the section goes back to
  a single utterance with the exact clock - the accurate path is unchanged.
- When a boundary lands mid-section, the remainder is spoken as one utterance
  (it used to skip to the next section).

podcast-test's engine stub finishes utterances now and pins the whole thing:
one sentence first, the next following on its own, the highlight inside the
spoken text, a boundary collapsing the remainder, the highlight advancing.
53 checks. Version 0.7.10, SW v102.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "freezmo",
-  "version": "0.7.9",
+  "version": "0.8.0",
   "private": true,
   "license": "GPL-3.0-only",
   "scripts": {