docs(frank-tts): retire the 5.5 s cold-voice claim everywhere it still lived

PR #15 corrected the engine's page, the research brief's law bullet and the
warmer's header, but the retired ladder had six more homes: both quadlet files
("6.2-8.0 s… 0.5 ms", "~6-8 s each", "paying the 5.5 s cold decode"),
`scripts/frank-tts-engine.env` (the whole law paragraph and the WARM_TEXT note),
`scripts/frank-tts.env`, the [[Frank TTS]] troubleshooting row that told an
operator to expect ~5.5 s, the research brief's consequences paragraph, and one
test comment.

All re-keyed to what was measured on the deployed container 2026-10-18: the
first synthesis of a freshly registered voice costs 1,883 ms against 1,226-1,283
ms afterwards, so the warm pass buys about 0.6 s — and the reason it exists at
all, which the pages never led with, is that the registry is memory-only: without
the hook every voice name is a 400 until somebody synthesizes. The trial's dated
worklog entry stays as written; the correction points at it.

No behavior change: comments, unit lore and docs only, so no cfhash move and no
image rebuild.

tests/frank_tts_engine.bats

@@ -510,8 +510,8 @@ PY
     # two GGUFs (1.2 GiB + 291 MB) are still being read and the HTTP thread has
     # not bound yet. The warmer is launched against an address with nothing on
     # it, the listener appears a beat later, and the requirement is that the
-    # warmer notices and does the work — an early exit here is the day's first
-    # caller paying 5.5 s of silence for a voice nobody warmed.
+    # warmer notices and does the work — an early exit leaves every voice
+    # unregistered, and the façade then answers 400 for all of them.
     port="$(free_port)"
     ( ENGINE_URL="http://127.0.0.1:$port" VOICES_DIR="$VOICES" HEALTH_WAIT=60 \
         bash "$WARM" > "$TEST_TMP/race.log" 2>&1; echo "$?" > "$TEST_TMP/race.rc" ) &