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/dictations/ce1_pluriel_nom_dictee_01.json

@@ -0,0 +1,20 @@
+{
+  "id": "ce1_pluriel_nom_dictee_01",
+  "level": "CE1",
+  "domain": "grammaire",
+  "rule_ids": ["regle_pluriel_nom"],
+  "difficulty": 1,
+  "target_text": "Les chats jouent dans le jardin.",
+  "words": [
+    { "text": "Les", "expects_capital": true },
+    { "text": "chats", "rule_id": "regle_pluriel_nom" },
+    { "text": "jouent" },
+    { "text": "dans" },
+    { "text": "le" },
+    { "text": "jardin." }
+  ],
+  "audio_id": "ce1_pluriel_nom_dictee_01",
+  "audio_provisional": true,
+  "accepted_variants": [],
+  "review_status": "draft"
+}