feat(tts): bump base speed 1.0→1.15 for slightly faster delivery

User asked for a bit faster speech. Pitch shift is already disabled, so the
earlier "monster voice" risk from stacking pitch on a fast base is gone — a
modest 1.15x base is natural. Emotion multipliers scale off base, so every
emotion gets the bump too. Also adds tests/gen_emotion_samples.py, a dev
utility that synthesizes one clip per canonical emotion (announce name at
neutral speed, then a sample sentence steered by that emotion's tag) and
stitches them into a single wav for auditioning the emotion palette.

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

wsai/backends/melo.py

@@ -12,7 +12,7 @@ Env:
     WSAI_MELO_DEVICE   cpu | cuda | auto  (default auto: GPU if torch sees one,
                        else CPU; the worker falls back to CPU if CUDA fails)
     WSAI_TTS_OUT_DIR   where wavs are written (default ~/.cache/wsai/tts)
-    WSAI_TTS_SPEED     synthesis speed multiplier (default 1.0)
+    WSAI_TTS_SPEED     synthesis speed multiplier (default 1.15)
 """
 
 from __future__ import annotations
@@ -93,7 +93,7 @@ class MeloTTS:
         self.out_dir = Path(out_dir or os.environ.get("WSAI_TTS_OUT_DIR")
                             or (Path.home() / ".cache/wsai/tts"))
         self.speed = float(speed if speed is not None
-                           else os.environ.get("WSAI_TTS_SPEED", "1.0"))
+                           else os.environ.get("WSAI_TTS_SPEED", "1.15"))
         self.sink = sink or _log_sink
         self._proc: asyncio.subprocess.Process | None = None
         self._lock = asyncio.Lock()