extract the production rules as free functions over config and building

gatherCandidateRecipes, recipeInputsAvailable, computeShipyardRequiredMaterials,
hasInputsToStart and getProductionStatus read no factory state — they answer
"what can this building produce, and can it start" from the config and the
Building alone. They move to ProductionRules.h as free functions, and the
ProductionStatus enum goes with them since it is that group's return type.

Two of the five are pure in their arguments; the other three need GameConfig
through gatherCandidateRecipes and computeShipyardRequiredMaterials, so config is
a parameter rather than the group being split across two headers.

Only two callers outside BuildingSystem existed — the status light in
GameWorldView and one test lambda — so this is nearly all internal.

Verified with a golden-checksum capture before and after — all four sample ticks
identical.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YHcUerKAZKWNvSKJxYKbnG

src/test/BuildingTest.cpp

@@ -1,5 +1,6 @@
 #include "catch.hpp"
 #include "FactoryQueries.h"
+#include "ProductionRules.h"
 
 #include <map>
 #include <random>
@@ -1562,7 +1563,7 @@ TEST_CASE("BuildingSystem: getProductionStatus classifies production state", "[b
     }
     REQUIRE_FALSE(shipId.empty());
 
-    const auto statusOf = [&f](const Building& b) { return f.bs.getProductionStatus(b); };
+    const auto statusOf = [&f](const Building& b) { return getProductionStatus(f.cfg, b); };
 
     SECTION("non-production buildings show no status light")
     {