Derive ship scrap drop from threat

src/test/ConfigLoaderTest.cpp

@@ -78,6 +78,7 @@ TEST_CASE("ConfigLoader loads the committed bin/config/ configs end-to-end", "[c
     REQUIRE(cfg.world.push.bossAdvanceSeconds == Approx(60.0));
     REQUIRE(cfg.world.orbitFactor == Approx(0.8));
     REQUIRE(cfg.world.rallyOrbitRadius_tiles == Approx(5.0));
+    REQUIRE(cfg.world.scrapPerThreat == Approx(1.0));
 
     // Spot-check that a config-derived formula computes as expected.
     // threat_rate_formula = "x": evaluates to the input value.
@@ -167,6 +168,7 @@ TEST_CASE("Missing field in world.toml is rejected with the field path", "[confi
 height_tiles = 60
 refund_percentage = 75
 scrap_despawn_seconds = 30
+scrap_per_threat = 0.01
 tile_size_m = 10
 belt_speed_mps = 20
 starting_building_blocks = 100
@@ -217,6 +219,7 @@ TEST_CASE("Malformed formula in world.toml is rejected with field identification
 height_tiles = 60
 refund_percentage = 75
 scrap_despawn_seconds = 30
+scrap_per_threat = 0.01
 tile_size_m = 10
 belt_speed_mps = 20
 starting_building_blocks = 100
@@ -268,6 +271,7 @@ TEST_CASE("Inverted wave gap range is rejected", "[config]")
 height_tiles = 60
 refund_percentage = 75
 scrap_despawn_seconds = 30
+scrap_per_threat = 0.01
 tile_size_m = 10
 belt_speed_mps = 20