games/kaintuck: surface boatyard credit on the cargo-buy screens

#42 in Riparion/riparion-retro — merged 2026-06-13

Problem

A new-game player reports: "if I buy the smallest ship, I still don't have money to buy cargo."

The mechanics already work — they were just hidden:

• Fresh house starts with $50 cash + $200 boatyard credit cap.
• Cheapest boat (Skiff $40) + 1 crew ($3) = $43, leaving $7 cash / $0 debt / $200 credit.
• Cargo max_buy is funded by cash + available credit, and the trade screen's Buy / "All" buttons use it — so the player can fill the 30-unit hold on credit (the intended "buy cheap on credit upstream, sell high downstream" loop).

But the BUY CARGO header showed only Cash $7, and the river status bar shows debt owed (= $0), never the credit still on tap. So the player sees $7 and concludes they're broke.

Fix (UI only)

• trade.rs: show available credit in the header on buy-capable screens — Cash $7 · credit $200 · hold 0/30. Sell-only screens (Natchez) keep the old cash-only header.
• pittsburgh.rs: empty-hold hint now reads "Buy cheap cargo on credit here to sell downriver."

No economy/engine changes, so scenario_matches_legacy_consts and golden_trace_is_stable stay green (verified locally: 49 passed, 0 failed).

🤖 Generated with Claude Code