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

world/resource_nodes/ResourceNode.gd

@@ -0,0 +1,100 @@
+class_name ResourceNode
+extends Node3D
+
+signal amount_changed(node_id: StringName, amount_remaining: float)
+signal depleted(node_id: StringName)
+signal reservation_changed(node_id: StringName, agent_id: int)
+
+static var _all: Array[ResourceNode] = []
+
+@export var node_id: StringName
+@export var action_id: StringName = &"gather_food"
+@export var resource_id: StringName = &"food"
+@export var amount_remaining: float = 10.0
+@export var yield_per_action: float = 2.0
+@export var enabled: bool = true
+@export var can_npcs_use: bool = true
+@export var can_player_use: bool = true
+@export var hide_visual_when_depleted: bool = false
+
+@onready var interaction_point: Marker3D = $InteractionPoint
+
+var reserved_by: int = -1
+
+func _ready():
+	if node_id.is_empty():
+		push_error("ResourceNode at %s has empty node_id" % get_path())
+	_all.append(self)
+	add_to_group("resource_nodes")
+	amount_changed.connect(_update_debug_label)
+	reservation_changed.connect(_update_debug_label)
+	_update_debug_label()
+
+func _exit_tree():
+	_all.erase(self)
+
+func is_depleted() -> bool:
+	return amount_remaining <= 0.0
+
+func can_extract() -> bool:
+	return enabled and not is_depleted()
+
+func is_available_for(agent_id: int) -> bool:
+	return enabled and not is_depleted() and (reserved_by == -1 or reserved_by == agent_id)
+
+func reserve(agent_id: int) -> bool:
+	if not is_available_for(agent_id):
+		return false
+	reserved_by = agent_id
+	reservation_changed.emit(node_id, agent_id)
+	return true
+
+func release(agent_id: int) -> void:
+	if reserved_by == agent_id:
+		reserved_by = -1
+		reservation_changed.emit(node_id, -1)
+
+func extract(requested_amount: float = yield_per_action) -> float:
+	if not can_extract():
+		return 0.0
+	var extracted := minf(requested_amount, amount_remaining)
+	amount_remaining -= extracted
+	amount_changed.emit(node_id, amount_remaining)
+	if is_depleted():
+		depleted.emit(node_id)
+		if hide_visual_when_depleted and has_node("Visual"):
+			$Visual.visible = false
+	return extracted
+
+func _update_debug_label(_a = null, _b = null) -> void:
+	if not has_node("DebugLabel"):
+		return
+	var label := $DebugLabel as Label3D
+	var text := "%s\n%s %.1f" % [node_id, resource_id, amount_remaining]
+	if reserved_by >= 0:
+		text += "\nheld:%d" % reserved_by
+	if is_depleted():
+		text += "\nDEPLETED"
+	label.text = text
+
+static func get_by_id(node_id: StringName) -> ResourceNode:
+	for node in _all:
+		if node.node_id == node_id:
+			return node
+	return null
+
+static func find_available(action: StringName, agent_id: int, from_position: Vector3) -> ResourceNode:
+	var best: ResourceNode
+	var best_dist := INF
+	for node in _all:
+		if node.action_id != action:
+			continue
+		if not node.can_npcs_use:
+			continue
+		if not node.is_available_for(agent_id):
+			continue
+		var dist := from_position.distance_squared_to(node.global_position)
+		if dist < best_dist:
+			best_dist = dist
+			best = node
+	return best