perf(brain): switch voice brain to Sonnet 5 with an always-on brevity rule

Sonnet 5 reaches first token ~0.4s sooner than Sonnet 4.5 but answers the
same voice prompt more verbosely (measured 46-54 vs ~28 output tokens),
which erased the win in total turn time. Add a cached, persona-independent
brevity system block that pulls output back to ~30 tokens, so the faster
first token becomes a faster, lower-variance whole reply.

Measured (16-round interleaved A/B, production-shaped call):
  sonnet-4-5      TTFT 1.26s  total 2.02s (tail 3.26s)  out 29
  sonnet-5+brev   TTFT 0.85s  total 1.70s (tail 2.28s)  out 30

- ClaudeBrain: default model claude-sonnet-5 + BREVITY block (cached with
  the persona prefix so a dashboard persona edit can't drop it).
- Defaults aligned: config.Settings.anthropic_model and the voice-server
  WSAI_BRAIN_MODEL default -> claude-sonnet-5.
- Dashboard: add claude-sonnet-5 to LLM_OPTIONS + JS label; fix stale
  restart hint.
- tests/latency_ab.py: reproducible model-latency A/B harness (reads
  CLAUDE_CREDENTIALS_PATH; makes live API calls, so not a pytest test).

Streaming TTS was intentionally not added: this bot answers in one sentence,
where sentence-level streaming has no overlap to exploit, and it would
require rearchitecting both the Python endpoint and the node playback.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

wsai/dashboard.py

@@ -649,7 +649,7 @@ class Dashboard:
 
     # -- live model switching (STT size / LLM model) --------------------- #
     STT_OPTIONS = ["tiny", "base", "small", "medium", "large-v3"]
-    LLM_OPTIONS = ["claude-haiku-4-5", "claude-sonnet-4-5"]
+    LLM_OPTIONS = ["claude-haiku-4-5", "claude-sonnet-4-5", "claude-sonnet-5"]
 
     def models_settings(self) -> dict:
         stt = self.stt
@@ -1193,7 +1193,7 @@ PAGE = r"""<!DOCTYPE html>
       <div id="mLoad" class="mload" style="display:none">
         <span class="spin"></span><span id="mLoadText">로딩 중…</span>
       </div>
-      <p class="cfghint" style="margin:8px 0 0">STT는 전환 시 모델을 다시 로드합니다(처음 medium/large-v3는 다운로드로 수 분 걸릴 수 있어요). LLM은 다음 답변부터 즉시 적용됩니다. 서비스 재시작 시 기본값(medium · Haiku)으로 돌아갑니다.</p>
+      <p class="cfghint" style="margin:8px 0 0">STT는 전환 시 모델을 다시 로드합니다(처음 medium/large-v3는 다운로드로 수 분 걸릴 수 있어요). LLM은 다음 답변부터 즉시 적용됩니다. 마지막으로 고른 모델은 재시작 후에도 유지됩니다(기본값 Sonnet 5).</p>
     </div>
   </section>
   <div class="demobar" id="demobar" style="display:none"></div>
@@ -1519,7 +1519,8 @@ function wireCollapse(toggleId, bodyId, caretId){
 const STT_LABEL = {tiny:'tiny (가장 빠름)', base:'base', small:'small (빠름)',
   medium:'medium (기본·정확)', 'large-v3':'large-v3 (최고 정확도)'};
 const LLM_LABEL = {'claude-haiku-4-5':'Haiku 4.5 (가장 빠름)',
-  'claude-sonnet-4-5':'Sonnet 4.5 (고품질·조금 느림)'};
+  'claude-sonnet-4-5':'Sonnet 4.5 (고품질·조금 느림)',
+  'claude-sonnet-5':'Sonnet 5 (기본 · 빠르고 고품질)'};
 function fillSel(sel, options, current, labels){
   sel.innerHTML='';
   options.forEach(o=>{ const el=document.createElement('option');