Creates the full buy/economy subsystem for a tactical FPS:
- EconomyManager (server/scripts/economy/economy_manager.gd): per-player
money tracking with starting 00, kill/round/objective rewards, spending
validation, escalating loss bonuses, and money_changed signal.
- BuyMenuHandler (server/scripts/economy/buy_menu_handler.gd): server-side
RPC validation of purchase requests — checks weapon validity, buy-zone
membership (scans BuyZone nodes in scene tree), affordability, then
deducts money and grants weapon via WeaponServer.give_weapon().
- BuyMenu (client/characters/weapon/buy_menu.gd): client-side Control UI
with styled panel, money display, weapon grid (name/cost/stats), numeric
key (1-4) and mouse-click purchase, server-driven affordability, and
auto-close after purchase.
- GameServer integration: creates EconomyManager + BuyMenuHandler in
_ready(), registers players for economy on spawn, awards kill rewards
via DamageProcessor.player_killed signal, and unregisters on despawn.
Economy rules: win=+,250, loss=,900 (+00/streak, cap ,900),
kill=+00, bomb_plant/defuse=+00. Weapon costs: pistol=00,
smg=,200, rifle=,700, shotgun=,900.
shawn