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>

tests/data/test_mission_positions.gd

@@ -0,0 +1,28 @@
+extends GutTest
+## Garde-fou contre le chevauchement de marqueurs sur la carte (signale
+## manuellement : "La clairiere" et "La cabane" se chevauchaient, voir
+## docs/specs/decisions.md). scenes/world/world_map.gd place chaque marqueur
+## a une position fractionnelle (map_position.x/y de data/missions/*.json) ;
+## sans ecart suffisant entre deux missions, leurs boutons se recouvrent a
+## l'ecran. Verifie l'ecart plutot que le rendu reel (non testable sans
+## ecran), avec une marge volontairement un peu permissive par rapport a la
+## grille 0.35 utilisee dans les donnees.
+
+const MIN_FRACTIONAL_GAP := 0.3
+
+
+func test_mission_markers_do_not_overlap() -> void:
+    var missions := ExerciseEngine.get_missions()
+    for i in missions.size():
+        for j in range(i + 1, missions.size()):
+            var a: Dictionary = missions[i]["map_position"]
+            var b: Dictionary = missions[j]["map_position"]
+            var dx: float = abs(float(a["x"]) - float(b["x"]))
+            var dy: float = abs(float(a["y"]) - float(b["y"]))
+            assert_true(
+                dx >= MIN_FRACTIONAL_GAP or dy >= MIN_FRACTIONAL_GAP,
+                (
+                    "%s et %s sont trop proches (dx=%s, dy=%s)"
+                    % [missions[i]["mission_id"], missions[j]["mission_id"], dx, dy]
+                ),
+            )