Replace eSpeak NG with Piper neural voice for dictation audio

Manual test reported the eSpeak NG dictation voice as unintelligible.
Formant synthesis is inherently mechanical; slowing it down doesn't
fix that. Switched tools/generate_dictation_audio.py to piper-tts
(fr_FR-siwis-medium neural voice, MIT/CC-BY 4.0, attribution in
assets/CREDITS.md). The voice model (~60MB) downloads once on demand
into the untracked .cache/piper-voices/, never at game runtime.

Adds piper-tts to tools/setup.sh's venv and a pyrightconfig.json
pointing pyright at .venv, needed once tools/ imports a real
third-party package (`make lint` was silently checking against the
wrong interpreter before).

Regenerated ce1_accord_adjectif_dictee_01.ogg with the new voice.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tools/setup.sh

@@ -1,7 +1,9 @@
 #!/usr/bin/env bash
 # Installe l'outillage de developpement : binaire Godot pinne, addon GUT,
-# environnement Python pour gdtoolkit. Rien de ceci n'est commite (voir .gitignore) :
-# ce script est la seule source de verite pour reconstruire l'outillage.
+# environnement Python pour gdtoolkit/pyright/piper-tts (ce dernier pour
+# tools/generate_dictation_audio.py). Rien de ceci n'est commite (voir
+# .gitignore) : ce script est la seule source de verite pour reconstruire
+# l'outillage.
 set -euo pipefail
 
 cd "$(dirname "$0")/.."
@@ -66,7 +68,7 @@ if [ ! -d ".venv" ]; then
     python3 -m venv .venv
 fi
 ./.venv/bin/pip install --quiet --upgrade pip
-./.venv/bin/pip install --quiet "gdtoolkit==${GDTOOLKIT_VERSION}" "pyright==${PYRIGHT_VERSION}"
+./.venv/bin/pip install --quiet "gdtoolkit==${GDTOOLKIT_VERSION}" "pyright==${PYRIGHT_VERSION}" piper-tts
 
 echo "Import du projet (enregistre les class_name globaux : SchemaValidator, DataLoader...)."
 "${GODOT_BIN}" --headless --path . --import >/dev/null