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>

assets/audio/dictations/ce1_accord_adjectif_dictee_01.ogg.import

@@ -0,0 +1,19 @@
+[remap]
+
+importer="oggvorbisstr"
+type="AudioStreamOggVorbis"
+uid="uid://nheq35mknxfw"
+path="res://.godot/imported/ce1_accord_adjectif_dictee_01.ogg-74739a16a08bfb592c5dfe269dc458a5.oggvorbisstr"
+
+[deps]
+
+source_file="res://assets/audio/dictations/ce1_accord_adjectif_dictee_01.ogg"
+dest_files=["res://.godot/imported/ce1_accord_adjectif_dictee_01.ogg-74739a16a08bfb592c5dfe269dc458a5.oggvorbisstr"]
+
+[params]
+
+loop=false
+loop_offset=0
+bpm=0
+beat_count=0
+bar_beats=4