feat(ui): add YAML-based UI string loading system (#409)

Adds UIStrings autoload that loads display text from a YAML file,
replacing hardcoded strings in HUD and interaction prompt. Copy team
can now author UI microcopy in client/data/ui-strings.yaml without
touching GDScript. Includes 38 strings across 5 categories and 16
gdUnit4 tests for the parser and lookup API.

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

client/ui/hud.gd

@@ -13,12 +13,12 @@ func _ready() -> void:
 func update_from_hud_data(data: Dictionary) -> void:
 	# Update perception mode display
 	if data.has("perception_mode"):
-		perception_label.text = "Mode: " + data.perception_mode.capitalize()
+		perception_label.text = UIStrings.get_text("hud.mode_label") + ": " + data.perception_mode.capitalize()
 
 	# Update time display (per D-031)
 	if data.has("time"):
-		time_label.text = "Time: " + data.time
+		time_label.text = UIStrings.get_text("hud.time_label") + ": " + data.time
 
 # Update player health (from player data, not hud_data)
 func update_health(health: int) -> void:
-	health_label.text = "Health: " + str(health)
+	health_label.text = UIStrings.get_text("hud.health_label") + ": " + str(health)

client/ui/interaction_prompt.gd

@@ -40,7 +40,7 @@ func _show_prompt(interaction: Dictionary) -> void:
 
 	# 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
+	var display_text: String = "%s - %s" % [UIStrings.get_text("interaction.prompt_prefix"), verb_label]
 
 	if _current_target_id != target_id or prompt_label.text != display_text:
 		prompt_label.text = display_text