feat(simulation): complete interaction and monologue pipelines (#413, #414, #415)

Three fixes to make the gameplay loop functional end-to-end:

- Add Interactable component to NPC spawn so E-prompt detection works
- Build monologue trigger system (enter_location + time_idle) with
  MonologueBuffer/MonologueState components, wire through ObserverSnapshot
  as current_monologue field, decode on client and display via HUD
- Change PlayerAction::Interact from unit to struct variant carrying
  optional target_entity_id and verb fields

Bumps protocol version from 4 to 5. Regenerates MessagePack fixtures.
All 200 tests pass (170 unit + 30 integration).

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

client/scripts/main.gd

@@ -22,6 +22,12 @@ func _process(_delta: float) -> void:
 		if world_renderer and world_renderer.has_method("update_from_state"):
 			world_renderer.update_from_state()
 
+		# Show monologue if server sent one this tick (#414)
+		if GameState.current_monologue != null and monologue_display:
+			var mono: Dictionary = GameState.current_monologue
+			monologue_display.show_monologue(mono.get("text", ""), mono.get("duration_seconds", 5.0))
+			GameState.current_monologue = null  # Consume — don't re-show next frame
+
 	# Track camera to player position every frame (D-015: locked, no panning)
 	# Camera2D smoothing handles interpolation — we just set the target
 	camera.global_position = GameState.player_position * Constants.TILE_SIZE
@@ -31,9 +37,15 @@ func _process(_delta: float) -> void:
 	for input in inputs:
 		if input.action == InputMapper.Action.INTERACT:
 			var target_id: int = interaction_prompt.get_interaction_target()
+			# Always send struct form for Interact (#415) — server expects named fields
 			if target_id >= 0:
 				input["action_data"] = {
 					"target_entity_id": target_id,
 					"verb": interaction_prompt.get_selected_verb(),
 				}
+			else:
+				input["action_data"] = {
+					"target_entity_id": null,
+					"verb": null,
+				}
 		SimBridge.send_input(input)