gamebook: allow GAMEBOOK_OUT_DIR override

.gitignore

@@ -123,3 +123,6 @@ tools/gamebook/build/
 
 # Generated PLIP phone gamebook pack (reproducible)
 tools/gamebook/build_phone/
+
+# Generated master gamebook narration pack (GAMEBOOK_OUT_DIR override)
+tools/gamebook/build_master/

tools/gamebook/build_gamebook.py

@@ -26,6 +26,7 @@ from __future__ import annotations
 
 import argparse
 import json
+import os
 import struct
 import subprocess
 import sys
@@ -40,7 +41,8 @@ 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"
+OUT_DIR = Path(os.environ.get("GAMEBOOK_OUT_DIR",
+                              REPO / "tools" / "gamebook" / "build"))
 DEFAULT_VOICE = "Thomas"
 EXCLUDE = {"zacus_demo"}            # dev demo, not part of the player library
 MAX_TILES = 6                       # the firmware shows a 2x3 tile grid