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/exercises/ce1/ce1_pluriel_nom_qcm_04.json

@@ -0,0 +1,17 @@
+{
+  "id": "ce1_pluriel_nom_qcm_04",
+  "level": "CE1",
+  "domain": "grammaire",
+  "skill": "accord_groupe_nominal",
+  "rule_id": "regle_pluriel_nom",
+  "difficulty": 1,
+  "type": "multiple_choice",
+  "instruction": "Choisis le bon mot.",
+  "question": "Des ... roulent sur la route.",
+  "answers": ["voiture", "voitures"],
+  "correct_answer": 1,
+  "hint": "Le determinant « des » indique plusieurs voitures.",
+  "explanation": "Pour former le pluriel d'un nom, on ajoute un -s a la fin : « voitures ».",
+  "review_tags": ["pluriel", "pluriel_nom"],
+  "review_status": "draft"
+}