feat: add ResourceNode system with target selection and reservation

Phase 1-2 of the ResourceNode migration plan:

- Create ResourceNode class (world/resource_nodes/) with extraction,
  reservation, nearest-available selection, and debug label
- Place 4 resource instances in main.tscn (2 berry bushes, 2 trees)
- Add target_id to SimNPC for tracking which node an NPC is using
- Add set_npc_target_id and release_npc_reservation to SimulationManager
  with cleanup on task change, completion, and death
- Update WorldViewManager to query available ResourceNodes first,
  falling back to zone markers with a logged warning

simulation/SimNPC.gd

@@ -27,6 +27,7 @@ var is_dead := false
 var task_duration := 0.0
 var task_progress := 0.0
 var task_complete := true
+var target_id: StringName = &""
 
 func _init(
 	_id: int,

simulation/SimulationManager.gd

@@ -59,13 +59,19 @@ func simulate_tick() -> void:
 
 	for npc in npcs:
 		var old_task := npc.current_task
+		var old_target := npc.target_id
 		var old_state := npc.task_state
 		var was_complete := npc.task_complete
 		var was_dead := npc.is_dead
 
 		npc.simulate_tick(village)
 
+		if old_task != npc.current_task and old_target != &"":
+			release_npc_reservation(npc.id)
+
 		if not was_dead and npc.is_dead:
+			if old_target != &"":
+				release_npc_reservation(npc.id)
 			npc_died.emit(npc)
 			npc_task_changed.emit(npc, old_task, npc.current_task)
 
@@ -79,6 +85,9 @@ func simulate_tick() -> void:
 			var completed_task := npc.current_task
 			var needs_immediate_food_after_gather := npc.should_eat_immediately_after_task()
 
+			if npc.target_id != &"":
+				release_npc_reservation(npc.id)
+
 			village.apply_npc_task(npc)
 			village_was_changed = true
 
@@ -129,6 +138,22 @@ func simulate_tick() -> void:
 	if DEBUG_LOGS:
 		print(village.get_summary())
 		
+func set_npc_target_id(npc_id: int, target_id: StringName) -> void:
+	for npc in npcs:
+		if npc.id == npc_id:
+			npc.target_id = target_id
+			return
+
+func release_npc_reservation(npc_id: int) -> void:
+	for npc in npcs:
+		if npc.id == npc_id:
+			if npc.target_id != &"":
+				var node := ResourceNode.get_by_id(npc.target_id)
+				if node != null:
+					node.release(npc.id)
+				npc.target_id = &""
+			return
+
 func notify_npc_arrived(npc_id: int) -> void:
 	for npc in npcs:
 		if npc.id == npc_id: