Add CargoFilter property to PieceState, auto-assigned at placement
by tracing relay chain back to production. TransferResolver now
enforces cargo-type filtering and uses forward-direction sorting
with cargo-aware distance calculations. Prevents cross-route
contamination on multi-cargo boards.
Level 3 restored to dual-cargo (Wood+Stone) with correct 10R+2K stock.
Two new solvability tests validate filter auto-assignment and chain
propagation. All 60 tests green.
Samuel Bouchet
3120d9835e