Lets a trusted local extension contribute a TTS engine that appears in the
Settings -> TTS Engine dropdown and is used by BOTH playback paths (voice-mode
auto-read in boot.js _speakResponse + the per-message Listen button in ui.js
speakMessage). Mirrors window.registerHermesSkin.
window.registerHermesTtsEngine({ id, label, synthesize(text, opts) })
-> Promise<ArrayBuffer|Blob> of audio; core coerces + plays through the same
<audio> lifecycle as the Edge engine (stop/rearm in voice mode).
Core owns selection, the dropdown <option> (label via textContent, never HTML),
and playback. Built-in ids (browser/edge/elevenlabs) are reserved; invalid
descriptors are rejected; empty/failed synth degrades gracefully. The settings
panel re-adds registered options on render (the markup hardcodes only built-ins).
Unblocks TTS-engine extensions (e.g. a local VOICEVOX engine). Docs in
docs/EXTENSIONS.md; CHANGELOG [Unreleased]. New regression test drives the real
registry via a Node harness (register/validation/reserved-guard/synth coercion).
Full suite: 10893 passed, 0 failed. Verified end-to-end in a live browser with a
companion VOICEVOX extension: engine registers, appears in the dropdown
(browser/edge/elevenlabs/voicevox), and the Listen button routes through the
registered synthesize (both VOICEVOX steps called).
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