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>

data/missions/mission_09_rocher_qui_repete.json

@@ -0,0 +1,9 @@
+{
+  "mission_id": "mission_09_rocher_qui_repete",
+  "title": "Le rocher qui repete",
+  "narrative_context": "Le vieux rocher de la foret repete une phrase, une seule fois, avant de se rendormir. Ecoute bien et ecris-la sans faute.",
+  "rule_id": "regle_accord_determinant",
+  "exercise_type": "dictation",
+  "map_position": { "x": 0.45, "y": 0.88 },
+  "review_status": "draft"
+}