feat(ui): economics monitor insert panel with placeholder data (#824)

New implant panel at implant/economics: system selector, 6-commodity
price table with trend indicators, GDP strip. Composed from D-169
component library. Ring buffer caches last 20 ticks per system.
Snapshot routing wired through snapshot_handler → GameState →
snapshot_consumers → economics_panel. Placeholder prices shown
until server ships EconomySnapshot (#822).

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

client/ui/hud.tscn

@@ -1,7 +1,8 @@
-[gd_scene load_steps=2 format=3 uid="uid://cq1y5w3hmxr8b"]
+[gd_scene load_steps=3 format=3 uid="uid://cq1y5w3hmxr8b"]
 
 [ext_resource type="Script" path="res://ui/hud.gd" id="1_hud"]
 [ext_resource type="PackedScene" path="res://ui/star_map.tscn" id="2_starmap"]
+[ext_resource type="PackedScene" path="res://ui/implant/economics_panel.tscn" id="3_econ"]
 
 [node name="HUD" type="Control"]
 layout_mode = 3
@@ -19,3 +20,8 @@ script = ExtResource("1_hud")
 [node name="StarMap" parent="." instance=ExtResource("2_starmap")]
 visible = false
 
+; #824: Economics Monitor — implant/economics INSERT panel. Toggled via E key from main.gd.
+; Composes ImplantPanel from the D-169 component library. Placeholder data until #822 ships.
+[node name="EconomicsPanel" parent="." instance=ExtResource("3_econ")]
+visible = false
+