feat: add navigation_failed signal with time-based stuck detection and simulation pipeline

NpcVisual:
- Add navigation_failed(sim_id) signal distinct from arrived_at_target
  so the simulation layer can distinguish genuine arrival from failure
- Replace integer stuck_counter with delta-based stuck_time and
  exported stuck_timeout (1.5s) for frame-rate-independent detection
- Add path_request_id / path_pending mechanism to reject stale
  async path results when a new target is set or NPC dies
- On empty path or stuck movement, emit navigation_failed instead
  of arriving; only emit arrived_at_target when truly close to target
- Reduce arrival_distance back to 0.6 now that navigation_failure
  is properly signalled (no longer needed as a workaround)
- clamp arrival_distance reference in apply_dead_visual_state

SimNPC:
- Remove retry_count (replaced by navigation_failed signal pathway)
- Move last_task assignment from set_task() to task completion in
  SimulationManager so it reflects the task that actually finished

SimulationManager:
- Add notify_npc_navigation_failed(): release reservation, record
  last_task, send NPC to wander for 2s, emit task_changed signal
- Add notify_npc_target_unavailable() as public alias of above
- Guard extraction with node.reserved_by == npc.id to prevent
  stale/illegitimate extraction when reservation is lost
- Guard fallback zone task completion (apply_npc_task) so it only
  fires for non-resource tasks (gather_food/wood with no target
  silently skip instead of applying zone values)
- Remove retry_count safety net (fully replaced by signal pipeline)

WorldViewManager:
- Connect navigation_failed signal in spawn_npc_visual()
- Skip target assignment for empty/idle/dead task states
- When no resource node available for gather_food/wood, call
  notify_npc_target_unavailable() so NPC wanders instead of idling
- Add _on_npc_visual_navigation_failed handler delegating to
  SimulationManager.notify_npc_navigation_failed()
- Remove retry_count references

world/world_view_manager.gd

@@ -71,6 +71,11 @@ func spawn_npc_visual(npc: SimNPC) -> void:
 	else:
 		push_error("WorldViewManager: NPCVisual has no arrived_at_target signal")
 
+	if visual.has_signal("navigation_failed"):
+		visual.navigation_failed.connect(_on_npc_visual_navigation_failed)
+	else:
+		push_error("WorldViewManager: NPCVisual has no navigation_failed signal")
+
 	active_npc_visuals[npc.id] = visual
 	debug_log("Spawned visual for %s at %s" % [npc.npc_name, npc.position])
 
@@ -83,18 +88,23 @@ func update_npc_targets() -> void:
 			continue
 
 		var visual = active_npc_visuals[npc.id]
+		var task = npc.current_task
+		if task in ["", "idle", "dead"]:
+			continue
+
 		var resource_pos = _try_get_resource_target(npc, npc.current_task)
 		if resource_pos != null:
 			visual.set_target_position(resource_pos)
 			continue
 
-		var task = npc.current_task
 		if task == "wander":
 			var offset = Vector3(randf_range(-6.0, 6.0), 0.0, randf_range(-6.0, 6.0))
 			visual.set_target_position(visual.global_position + offset)
 			continue
 
 		if task in ["gather_food", "gather_wood"]:
+			if simulation_manager.has_method("notify_npc_target_unavailable"):
+				simulation_manager.notify_npc_target_unavailable(npc.id)
 			continue
 
 		var target = get_target_for_task(task)
@@ -137,7 +147,7 @@ func _try_get_resource_target(npc: SimNPC, task: String) -> Variant:
 
 	var node := ResourceNode.find_available(action_id, npc.id, visual.global_position)
 	if node == null:
-		debug_log("No available resource node for %s, using zone fallback" % task)
+		debug_log("No available resource node for %s" % task)
 		return null
 
 	if not node.reserve(npc.id):
@@ -145,7 +155,6 @@ func _try_get_resource_target(npc: SimNPC, task: String) -> Variant:
 		return null
 
 	npc.target_id = node.node_id
-	npc.retry_count = 0
 	debug_log("%s reserved %s for %s" % [npc.npc_name, node.node_id, task])
 	return node.interaction_point.global_position
 
@@ -155,6 +164,9 @@ func _on_npc_task_changed(npc: SimNPC, old_task: String, new_task: String) -> vo
 	
 	if not active_npc_visuals.has(npc.id):
 		return
+
+	if new_task in ["", "idle", "dead"]:
+		return
 	
 	var visual = active_npc_visuals[npc.id]
 	var resource_pos = _try_get_resource_target(npc, new_task)
@@ -168,6 +180,10 @@ func _on_npc_task_changed(npc: SimNPC, old_task: String, new_task: String) -> vo
 		return
 	
 	if new_task in ["gather_food", "gather_wood"]:
+		if simulation_manager.has_method("notify_npc_target_unavailable"):
+			simulation_manager.notify_npc_target_unavailable(npc.id)
+		else:
+			push_error("WorldViewManager: SimulationManager cannot handle unavailable targets")
 		return
 	
 	var target := get_target_for_task(new_task)
@@ -186,8 +202,19 @@ func _on_npc_visual_arrived(sim_id: int) -> void:
 		simulation_manager.notify_npc_arrived(sim_id)
 	else:
 		push_error("WorldViewManager: SimulationManager has no notify_npc_arrived method")
-		
-		
+
+func _on_npc_visual_navigation_failed(sim_id: int) -> void:
+	debug_log("NPC visual navigation failed. sim_id=%s" % sim_id)
+
+	if simulation_manager == null:
+		push_error("WorldViewManager: simulation_manager missing during navigation failure")
+		return
+
+	if simulation_manager.has_method("notify_npc_navigation_failed"):
+		simulation_manager.notify_npc_navigation_failed(sim_id)
+	else:
+		push_error("WorldViewManager: SimulationManager has no notify_npc_navigation_failed method")
+
 func _on_npc_died(npc: SimNPC) -> void:
 	if not active_npc_visuals.has(npc.id):
 		return