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/ui/interaction_prompt.gd

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+extends PanelContainer
+
+# Interaction prompt — displays available interaction verb for nearby entity.
+# Server-driven: shows when GameState.nearby_interactions is non-empty,
+# hides when empty. No game logic — pure display layer.
+#
+# v0.1: Single-line "E - Talk" (first verb on nearest entity)
+# v0.2: Will be replaced/extended with radial verb menu.
+# Public interface: get_interaction_target(), get_selected_verb()
+
+@onready var prompt_label: Label = $MarginContainer/PromptLabel
+
+var _is_showing: bool = false
+var _active_tween: Tween = null
+var _current_target_id: int = -1
+
+const FADE_IN: float = 0.15
+const FADE_OUT: float = 0.15
+
+func _ready() -> void:
+	modulate.a = 0.0
+	visible = false
+	_is_showing = false
+
+func _process(_delta: float) -> void:
+	var interactions: Array = GameState.nearby_interactions
+	if interactions.size() > 0:
+		_show_prompt(interactions[0])
+	elif _is_showing:
+		_hide_prompt()
+
+func _show_prompt(interaction: Dictionary) -> void:
+	var target_id: int = interaction.get("entity_id", -1)
+	var verbs: Array = interaction.get("verbs", [])
+
+	if verbs.is_empty():
+		if _is_showing:
+			_hide_prompt()
+		return
+
+	# v0.1: pick first verb (sorted by priority from server)
+	var verb_label: String = verbs[0].get("label", "")
+	var display_text: String = "E - %s" % verb_label
+
+	if _current_target_id != target_id or prompt_label.text != display_text:
+		prompt_label.text = display_text
+		_current_target_id = target_id
+
+	if not _is_showing:
+		visible = true
+		_is_showing = true
+		if _active_tween and _active_tween.is_valid():
+			_active_tween.kill()
+		_active_tween = create_tween()
+		_active_tween.tween_property(self, "modulate:a", 1.0, FADE_IN)
+
+func _hide_prompt() -> void:
+	if not _is_showing:
+		return
+	_is_showing = false
+	_current_target_id = -1
+	if _active_tween and _active_tween.is_valid():
+		_active_tween.kill()
+	_active_tween = create_tween()
+	_active_tween.tween_property(self, "modulate:a", 0.0, FADE_OUT)
+	_active_tween.tween_callback(func(): visible = false)
+
+## Returns the current interaction target entity ID, or -1 if no interaction.
+func get_interaction_target() -> int:
+	return _current_target_id
+
+## Returns the selected verb kind (v0.1: first verb on nearest, v0.2: radial selection).
+func get_selected_verb() -> String:
+	var interactions: Array = GameState.nearby_interactions
+	if interactions.is_empty():
+		return ""
+	var verbs: Array = interactions[0].get("verbs", [])
+	if verbs.is_empty():
+		return ""
+	return verbs[0].get("kind", "")

client/ui/interaction_prompt.gd.uid

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+uid://b43jk3eu2uig

client/ui/interaction_prompt.tscn

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+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://ui/interaction_prompt.gd" id="1_prompt"]
+
+[node name="InteractionPrompt" type="PanelContainer"]
+anchors_preset = 7
+anchor_top = 1.0
+anchor_right = 1.0
+anchor_bottom = 1.0
+offset_left = 200.0
+offset_top = -60.0
+offset_right = -200.0
+grow_horizontal = 2
+grow_vertical = 0
+mouse_filter = 2
+script = ExtResource("1_prompt")
+
+[node name="MarginContainer" type="MarginContainer" parent="."]
+layout_mode = 2
+theme_override_constants/margin_left = 16
+theme_override_constants/margin_top = 8
+theme_override_constants/margin_right = 16
+theme_override_constants/margin_bottom = 8
+
+[node name="PromptLabel" type="Label" parent="MarginContainer"]
+layout_mode = 2
+horizontal_alignment = 1
+text = ""