feat(gamebook): Kyutai backend for master gen

Add --tts say|kyutai to build_gamebook.py (the screen/BOX-3 generator),
mirroring the phone one: POST to the local Kyutai :8302 server, resample
24->16k via afconvert. Lets the BOX-3 use the nicer neural FR voice.

tools/gamebook/build_gamebook.py

@@ -29,12 +29,15 @@ import json
 import struct
 import subprocess
 import sys
+import tempfile
 import unicodedata
 import urllib.request
 from pathlib import Path
 
 import yaml
 
+KYUTAI_URL = "http://127.0.0.1:8302/tts"   # local Kyutai TTS 1.6B en_fr (MLX)
+
 REPO = Path(__file__).resolve().parents[2]
 BOOKS_DIR = REPO / "game" / "gamebooks"
 OUT_DIR = REPO / "tools" / "gamebook" / "build"
@@ -70,10 +73,36 @@ def clean_wav(raw: bytes) -> bytes:
             + b"data" + struct.pack("<I", len(pcm)) + pcm)
 
 
-def say_wav(text: str, voice: str, out: Path, force: bool) -> bool:
+def _kyutai_tts(text: str, out: Path) -> bool:
+    """Local Kyutai TTS 1.6B (en_fr) → 24 kHz WAV, resampled to 16 kHz mono via
+    afconvert (native macOS), then canonicalised. High-quality French voice."""
+    req = urllib.request.Request(
+        KYUTAI_URL, data=json.dumps({"text": text}).encode(),
+        headers={"Content-Type": "application/json"}, method="POST")
+    raw = urllib.request.urlopen(req, timeout=300).read()   # synth is slow
+    with tempfile.NamedTemporaryFile(suffix=".wav", delete=False) as tmp:
+        tmp.write(raw)
+        tmp_path = tmp.name
+    subprocess.run(["afconvert", "-f", "WAVE", "-d", "LEI16@16000", "-c", "1",
+                    tmp_path, str(out)], check=True, capture_output=True, timeout=60)
+    Path(tmp_path).unlink(missing_ok=True)
+    out.write_bytes(clean_wav(out.read_bytes()))
+    return out.stat().st_size > 44
+
+
+def say_wav(text: str, voice: str, out: Path, force: bool,
+            backend: str = "say") -> bool:
+    """Synthesise to a 16 kHz mono WAV. Cached by filename (resumable);
+    backend "kyutai" uses the local neural FR voice, else macOS say."""
     if out.exists() and out.stat().st_size > 44 and not force:
         return True
     out.parent.mkdir(parents=True, exist_ok=True)
+    if backend == "kyutai":
+        try:
+            return _kyutai_tts(text, out)
+        except Exception as exc:  # noqa: BLE001
+            print(f"  ! kyutai failed: {str(exc)[:140]}", file=sys.stderr)
+            return False
     for v in (voice, DEFAULT_VOICE):
         try:
             subprocess.run(
@@ -115,7 +144,8 @@ def narration(text: str, choices: list[dict]) -> str:
 
 
 def build_book(path: Path, force: bool, dry: bool, ip: str,
-               json_only: bool = False, narrate_text: bool = False) -> dict | None:
+               json_only: bool = False, narrate_text: bool = False,
+               tts: str = "say") -> dict | None:
     """Synthesise + (optionally) stage one book. Returns its library entry."""
     bid = path.stem
     book = yaml.safe_load(path.read_text())
@@ -139,7 +169,7 @@ def build_book(path: Path, force: bool, dry: bool, ip: str,
             # as tappable buttons, so they don't need to be spoken).
             spoken = (" ".join(p["text"].split()) if narrate_text
                       else narration(p["text"], choices))
-            if not say_wav(spoken, voice, out, force):
+            if not say_wav(spoken, voice, out, force, tts):
                 print(f"  ✗ synth {bid}/{pid}"); n_fail += 1; continue
         n_ok += 1
         manifest["passages"][pid] = {
@@ -176,6 +206,8 @@ def main() -> int:
     ap.add_argument("--narrate-text", action="store_true",
                     help="WAV narration = passage text only (no spoken choices) — for the BOX-3")
     ap.add_argument("--only", default="", help="comma-separated book ids to build")
+    ap.add_argument("--tts", choices=["say", "kyutai"], default="say",
+                    help="TTS backend (kyutai = local neural FR, much nicer/slower)")
     args = ap.parse_args()
 
     books = sorted(p for p in BOOKS_DIR.glob("*.yaml") if p.stem not in EXCLUDE)
@@ -190,7 +222,7 @@ def main() -> int:
     fail = 0
     for path in build_books:
         e = build_book(path, args.force, args.dry_run, args.ip, args.json_only,
-                       args.narrate_text)
+                       args.narrate_text, args.tts)
         if e is None:
             fail += 1
             continue