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/curriculum/ce1_accords_groupe_nominal.json

@@ -3,6 +3,6 @@
   "region_id": "foret_des_accords",
   "chapter_id": "accords_groupe_nominal",
   "title": "Les accords dans le groupe nominal",
-  "rule_ids": ["regle_accord_adjectif"],
+  "rule_ids": ["regle_accord_adjectif", "regle_pluriel_nom", "regle_accord_determinant"],
   "review_status": "draft"
 }