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/scenes/main.tscn

@@ -1,4 +1,4 @@
-[gd_scene load_steps=9 format=3 uid="uid://bswrmh7w8dbgm"]
+[gd_scene load_steps=10 format=3 uid="uid://bswrmh7w8dbgm"]
 
 [ext_resource type="Script" path="res://scripts/main.gd" id="1_main"]
 [ext_resource type="Script" path="res://scripts/rendering/world_renderer.gd" id="2_world"]
@@ -8,6 +8,7 @@
 [ext_resource type="PackedScene" path="res://ui/hud.tscn" id="6_hud"]
 [ext_resource type="PackedScene" path="res://ui/minimap.tscn" id="7_minimap"]
 [ext_resource type="PackedScene" path="res://ui/monologue_display.tscn" id="8_monologue"]
+[ext_resource type="PackedScene" path="res://ui/interaction_prompt.tscn" id="9_prompt"]
 
 [node name="Game" type="Node2D"]
 script = ExtResource("1_main")
@@ -36,3 +37,5 @@ zoom = Vector2(2, 2)
 [node name="Minimap" parent="UILayer" instance=ExtResource("7_minimap")]
 
 [node name="MonologueDisplay" parent="UILayer" instance=ExtResource("8_monologue")]
+
+[node name="InteractionPrompt" parent="UILayer" instance=ExtResource("9_prompt")]