Completes the trade-richness simulation logic. New DiplomaticAgreement::ResourceSale
forms as a barter FALLBACK: when a pair can't swap (one side has a surplus the
other lacks but not vice-versa), the surplus holder SELLS the resource to the
buyer for SALE_GOLD_PER_TURN. evaluate_trades produces it after the swap passes
(sale_candidate probes pa-then-pb, luxuries-then-strategics, deterministic).
TradeLedger.gold_flow_for(player) exposes the per-turn flow (seller +, buyer −);
incoming_luxuries/incoming_strategics route the bought resource to the right pool;
has_resource_sale + breaks-on-war. Variant grouped with the swap arms across the
renewal/courier matches; break_trades_on_war gets its own ResourceSale arm.
Verified: 4 new cargo tests (forms-as-fallback, no-sale-when-swap, strategic-
routing, breaks-on-war); existing no-surplus test updated for the new fallback;
mc-trade 66/0; api-gdext + mc-turn compile.
p3-23 stays partial — both trade-logic halves (swaps + sales) now done + tested;
remaining is the in-game application (mc-economy gold flow + traded-resource
gating/happiness + GDScript deal UI), which lands with p3-24's economy port.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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