Update PLAN.md: mark Phase 2 as done

PLAN.md

@@ -7,30 +7,18 @@
 - Relay chain mechanic with shared relay points (collision-free alternating)
 - Transfer system: production → pieces → demands, 4-adjacency, participation tracking
 - Victory/defeat: all demands met vs deadline expired
-- 57 tests passing: unit, solvability, full-level (Levels 1-3)
 
-### Key findings from Phase 1
+## Phase 2: Cargo-type aware transfers (DONE)
 
-- **Transfer direction bug fixed**: receiver priority in piece-to-piece transfers now
-  prefers pieces farther from production (pushes cargo forward, not backward).
-- **GDD stock corrections**: Level 2 needs 6R+1B (GDD had 4R+1B), Level 3 simplified
-  to single cargo type with 6R+1K (GDD had 4R+1B+2K for dual-cargo, which is infeasible).
-- **Cross-route contamination**: TransferResolver has no cargo-type filtering — adjacent
-  pieces from different routes transfer cargo regardless of type. On a 6x6 board, two
-  diagonal routes cannot avoid cross-adjacency. Dual-cargo levels require engine support.
-
-## Phase 2: Cargo-type aware transfers
-
-**Goal**: Enable multi-route, multi-cargo-type levels on small boards.
-
-- Add optional cargo-type filter to pieces (or routes), so a piece configured for Wood
-  ignores adjacent Stone and vice-versa.
-- Alternative: transfer resolver checks if cargo type matches the demand reachable from
-  the receiver's relay chain (more complex, automatic).
-- Simplest approach: production only gives to pieces whose relay chain leads toward a
-  compatible demand. Requires route/chain tracking.
-- Test: reintroduce Level 3 dual-cargo variant (Wood + Stone crossing 6x6 board with
-  L-shaped wall).
+- `CargoFilter` property on `PieceState`: optional `CargoType?` restricting accepted cargo
+- Auto-assigned at placement via relay chain tracing (adjacency to production, then shared
+  relay points with filtered pieces)
+- `TransferResolver` enforces filter: receivers with mismatched `CargoFilter` are skipped
+- Forward-direction sorting uses cargo-type-aware `MinDistanceToProduction` to avoid
+  wrong sorting when multiple productions exist
+- Level 3 restored to dual-cargo (Wood+Stone) with 10R+2K stock
+- GDD stock corrections: Level 2 = 6R+1B, Level 3 = 10R+2K
+- 60 tests passing including 2 new CargoFilter tests
 
 ## Phase 3: Surplus stock and puzzle difficulty tuning