fix(frank-tts): answer a voice-less request with the declared default

The omp speech device forwards a voice only when the call passes voice_id - there is no setting that feeds one (speech.voice is the TUI speak-aloud Kokoro enum, tts.localVoice belongs to the local backend) - so every harness voiceover arrives with no voice key and the shim answered 400 "unknown voice None". Measured on the deployed endpoint 2026-10-18, from a restarted session with the speech role pointed here.

DEFAULT_VOICE makes the absent case a spoken operator choice instead of a code surprise: the pack named there answers a voice-less request and the journal line is marked (default), so an unrequested voice never reads later like a requested one. An UNKNOWN name still 400s with the list that exists - the no-silent-fallback law that this endpoint was built on is untouched, and the strict shape stays the code default (empty DEFAULT_VOICE), which the second new seam test pins. Ships narrator1 in scripts/frank-tts.env.

scripts/frank-tts.env

@@ -57,6 +57,16 @@ TTS_BACKEND=omni-http
 TTS_UPSTREAM=http://frank-tts-engine:8880
 TTS_UPSTREAM_MODEL=voxcpm2
 
+# THE DEFAULT VOICE — what a request that names NO voice speaks. The omp harness
+# has no setting that feeds a voice to its speech device (`voice_id` is a
+# per-call argument; `speech.voice` belongs to the TUI's Kokoro speak-aloud and
+# never reaches an HTTP request), so a harness voiceover arrives voice-less and
+# would otherwise be a 400 for every caller that does not name one. An UNKNOWN
+# name still 400s with the list that exists — this covers the absent case only —
+# and the journal marks such a line `(default)` so an unrequested voice never
+# reads as a requested one. Any pack directory in /home/dank/voices is legal.
+DEFAULT_VOICE=narrator1
+
 # CLI-backend knobs, inert while TTS_BACKEND=omni-http. Empty means "not
 # configured": the endpoint answers a 500 that says so instead of running
 # `nice env -m ...` with an empty binary name.