feat(client): add interaction prompt system (#405)

Server-driven interaction prompt that displays "E - Talk" when near
an interactable NPC. Decodes v4 nearby_interactions from snapshot,
stores in GameState, renders via InteractionPrompt UI with fade
animation. Extensible interface (get_interaction_target/get_selected_verb)
for future radial verb menu (v0.2).

- Protocol: decode nearby_interactions array with nested VerbOption structs,
  entity relationship/observation fields, tick_rate in GameTime
- GameState: store/clear nearby_interactions per snapshot
- SimBridge: test mode generates v4 format with structured verbs
- InteractionPrompt: PanelContainer with fade in/out, polls GameState
- Tests: 19 new test cases covering protocol, state, sim bridge, UI, encoding
- Fixture assertions updated for v4 protocol version

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/main.gd

@@ -4,6 +4,7 @@ extends Node2D
 @onready var camera = $Camera2D
 @onready var hud = $UILayer/HUD
 @onready var monologue_display = $UILayer/MonologueDisplay
+@onready var interaction_prompt = $UILayer/InteractionPrompt
 
 func _ready() -> void:
 	print("The Settled Reach — client initialized")
@@ -28,4 +29,12 @@ func _process(_delta: float) -> void:
 	# Send queued input to simulation
 	var inputs = InputMapper.flush_queue()
 	for input in inputs:
+		# TODO: Once server accepts Interact(InteractData), attach target + verb:
+		#   if input.action == InputMapper.Action.INTERACT:
+		#       var target_id: int = interaction_prompt.get_interaction_target()
+		#       if target_id >= 0:
+		#           input["action_data"] = {
+		#               "target_entity_id": target_id,
+		#               "verb": interaction_prompt.get_selected_verb(),
+		#           }
 		SimBridge.send_input(input)