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/scripts/autoloads/game_state.gd

@@ -116,6 +116,12 @@ var ai_enhanced_dialogue_enabled: bool = true
 # "full" response hydrates ai_enhanced_dialogue_enabled (server is authoritative for persisted state).
 var settings_response: Variant = null
 
+# v21 fields (#824, D-181): Economy snapshot from server.
+# Dictionary keyed by system_id → { price_current, price_trend, trade_flow_volume,
+# corporate_presence, stockpile_weeks, production_vs_baseline, official_coverage_ratio }
+# Null when no economy data in the current snapshot.
+var economy_snapshot: Variant = null
+
 # v7 fields (#431, D-059/D-060)
 var pending_recognitions: Array = []  # [{entity_id, x, y, z, remaining_ticks, total_delay_ticks}]
 

client/scripts/autoloads/hud_groups.gd

@@ -14,6 +14,7 @@ extends Node
 ##   "implant/map/starchart"   — star map navigator
 ##   "implant/wiki/gttr"       — Drifter's Guide reader
 ##   "implant/journal"         — knowledge journal
+##   "implant/economics"       — economics monitor (D-181, #824)
 ##
 ## Usage:
 ##   HudGroups.register(self, "implant/map/starchart")