Add regle_pluriel_nom and regle_accord_determinant (CE1)

Extends explicitly beyond the §15 prototype scope (3 missions, one
rule) that prompt-lancement.md says not to exceed without flagging
it — confirmed with the user before implementing (decisions.md,
point 25).

Adds the two remaining CE1 rules for the "accords dans le groupe
nominal" chapter listed in prompt-lancement.md, each with the same
minimum content volume already established for regle_accord_adjectif
(8 multiple-choice, 4 sentence corrections, 3 dictées across the 3
difficulty levels) and 3 missions (one per exercise type, matching
the existing pattern), bringing the map to 9 locations.

world_map.gd is purely data-driven already, so no code change was
needed to support more locations — only its stale "trois lieux"
header comment, corrected. Dictation audio generated with the Piper
voice (make generate-dictation-audio).

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

assets/audio/dictations/ce1_pluriel_nom_dictee_03.ogg.import

@@ -0,0 +1,19 @@
+[remap]
+
+importer="oggvorbisstr"
+type="AudioStreamOggVorbis"
+uid="uid://bsc15pmxhbncx"
+path="res://.godot/imported/ce1_pluriel_nom_dictee_03.ogg-4505c6140e2a079498b6081b5c01d590.oggvorbisstr"
+
+[deps]
+
+source_file="res://assets/audio/dictations/ce1_pluriel_nom_dictee_03.ogg"
+dest_files=["res://.godot/imported/ce1_pluriel_nom_dictee_03.ogg-4505c6140e2a079498b6081b5c01d590.oggvorbisstr"]
+
+[params]
+
+loop=false
+loop_offset=0
+bpm=0
+beat_count=0
+bar_beats=4