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/rules/regle_accord_determinant.json

@@ -0,0 +1,9 @@
+{
+  "rule_id": "regle_accord_determinant",
+  "level": "CE1",
+  "domain": "grammaire",
+  "skill": "accord_groupe_nominal",
+  "statement": "Le determinant (le, la, les, un, une, des...) s'accorde en genre et en nombre avec le nom qu'il accompagne.",
+  "example": "Le chat. La fleur. Les chats. Les fleurs.",
+  "review_status": "draft"
+}

data/rules/regle_pluriel_nom.json

@@ -0,0 +1,9 @@
+{
+  "rule_id": "regle_pluriel_nom",
+  "level": "CE1",
+  "domain": "grammaire",
+  "skill": "accord_groupe_nominal",
+  "statement": "Le plus souvent, on forme le pluriel d'un nom en ajoutant un -s a la fin du mot.",
+  "example": "Un chat. Des chats. Une fleur. Des fleurs.",
+  "review_status": "draft"
+}