Generate provisional dictation audio so dictée missions can be played

Manual testing confirmed the "La clairière de l'écho" dictée mission
was unplayable: no audio file existed yet (decisions.md point 8), so
there was nothing to listen to before transcribing the sentence.

Adds tools/generate_dictation_audio.py (offline eSpeak NG synthesis +
ffmpeg encoding to Ogg Vorbis, exposed as `make
generate-dictation-audio`) and uses it to generate
ce1_accord_adjectif_dictee_01.ogg. A new headless test
(tests/data/test_dictation_audio.gd) guards against future dictées
being added without regenerating their audio.

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

Makefile

@@ -6,7 +6,7 @@ PYRIGHT := $(VENV)/bin/pyright
 PYTHON := $(VENV)/bin/python3
 GD_SOURCES := autoload scripts tests
 
-.PHONY: setup run lint format validate-data test check content-review export-linux export-windows
+.PHONY: setup run lint format validate-data test check content-review generate-dictation-audio export-linux export-windows
 
 setup:
 	tools/setup.sh
@@ -32,6 +32,9 @@ check: lint validate-data test
 content-review:
 	$(PYTHON) tools/generate_content_review.py
 
+generate-dictation-audio:
+	$(PYTHON) tools/generate_dictation_audio.py
+
 export-linux:
 	mkdir -p build/linux
 	$(GODOT_BIN) --headless --path . --export-release "Linux" build/linux/aventuriers_des_mots.x86_64