- Introduce CargoHold (pure class) as the ship's trade-goods carrier: fixed bays, one good each, capacity = unlocked bays, save/restore round-trip. Ship.cargo + setCargoHold/cargoRoom wired into stats.
- Add three credit-costed cargo builds (cargo-bay-8/12/16) that expand the hold 4→8→12→16 (MAX wins). BuildWindow affordability now covers both minerals and credits; GameScene.beginBuild debits credit lines and _applyCargoUpgrade expands the hold on completion.
- Add tradePreview/resultingCargo to Economy.js as the shop's pure calculator: sell/buy pricing, net credits, the two caps (can't go negative, end hold fits capacity), and atomic sell-to-free-room-for-buy.
- New ShopView + TradeGrid compose the buy/sell screen over the shop clip: SELL grid of cargo bays (tap to check), BUY grid with quantity steppers, live NET preview, Confirm/Clear, and a session-local Buyback tab (neutral undo at the sale price, locked until a sale). Wired into SurfaceScene's SHOP button.
- data/shopui.json drives the shop layout/animation knobs; economy.json gains cargo.maxSlots (16-slot grid); ship.json base cargoHold drops to 4 unlocked bays.
- Headless tests: dev/cargo.test.mjs (hold rules + trade preview caps), dev/shop-ui.test.mjs (construct, sell, buy, caps, buyback, clear), plus updated builds/ship-behavior suites; phaser-stub gains Geom.Rectangle for the hitArea path.
