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/autoloads/sim_bridge.gd

@@ -294,20 +294,34 @@ func _test_snapshot() -> Dictionary:
 			"visibility": sector,
 		})
 
+	# v4: nearby_interactions when NPC is nearby and visible (#404/#405)
+	var nearby: Array = []
+	if npc_dist <= 2 and _test_has_los(Vector2i(px, py), npc_pos):
+		nearby.append({
+			"entity_id": 2,
+			"entity_type": "Npc",
+			"distance": npc_dist,
+			"verbs": [
+				{"kind": "Talk", "label": "Talk", "priority": 0, "available": true},
+				{"kind": "ExamineNpc", "label": "Examine", "priority": 1, "available": true},
+			],
+		})
+
 	return {
 		"tick": _test_tick,
-		"version": 2,
+		"version": 4,
 		"game_time": {
 			"day": 0,
 			"time_of_day": _test_tick * 10,
 			"day_phase": "Morning",
-			"paused": false,
+			"tick_rate": "Full",
 		},
 		"player_facing": _test_facing,
 		"entities": entities,
 		"tiles": _test_tiles(),
 		"visible_tiles": _test_visible_tiles(),
 		"visible_positions": _test_visible_positions(),
+		"nearby_interactions": nearby,
 	}
 
 # Generate a small test room: 8x6 room with walls, a door, and floor