Fix overlapping mission markers on the world map

Manual test: "La clairière" and "La cabane" overlapped on screen.
Their fractional positions (0.8,0.65) and (0.92,0.78) were too close
given the marker size at the project's default window size — exactly
the risk flagged but unverified in the previous commit.

Repositioned all 9 missions (not just the new ones) onto a regular
3x3 grid with 0.35 spacing, well clear of marker dimensions at the
default resolution. Added a headless regression test
(tests/data/test_mission_positions.gd) that fails if any two
missions are closer than a 0.3 fractional gap on both axes — a
data-level guard, not a substitute for visually checking actual
on-screen layout.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

data/missions/mission_06_mare_grenouilles.json

@@ -4,6 +4,6 @@
   "narrative_context": "Les grenouilles de la mare chantent une phrase, une seule fois. Ecoute-les bien et ecris exactement ce qu'elles chantent.",
   "rule_id": "regle_pluriel_nom",
   "exercise_type": "dictation",
-  "map_position": { "x": 0.65, "y": 0.15 },
+  "map_position": { "x": 0.85, "y": 0.15 },
   "review_status": "draft"
 }