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/game_state.gd

@@ -19,6 +19,9 @@ var visibility_sectors: Dictionary = {}  # Vector2i -> "Forward"/"Peripheral"
 # refined when the server assigns explicit player entity IDs).
 var player_entity_id: int = 1
 
+# v4 fields (#404/#405)
+var nearby_interactions: Array = []  # [{entity_id, entity_type, distance, verbs: [{kind, label, priority, available}]}]
+
 func apply_snapshot(snapshot: Dictionary) -> void:
 	current_snapshot = snapshot
 
@@ -54,6 +57,12 @@ func apply_snapshot(snapshot: Dictionary) -> void:
 	if snapshot.has("player_facing") and snapshot.player_facing is String:
 		player_facing = snapshot.player_facing
 
+	# v4: nearby_interactions (#404/#405)
+	if snapshot.has("nearby_interactions") and snapshot.nearby_interactions is Array:
+		nearby_interactions = snapshot.nearby_interactions
+	else:
+		nearby_interactions = []
+
 	# v2: visible_tiles with visibility sectors
 	# Derives visible_positions when not explicitly provided (real server mode)
 	if snapshot.has("visible_tiles") and snapshot.visible_tiles is Array and snapshot.visible_tiles.size() > 0: