implement unlock dependencies

bin/app/data/config/ships.toml

@@ -4,6 +4,11 @@
 # content; stats, materials, and production times are placeholders until the
 # recipe and balancing passes.
 #
+# Unlock progression is mostly disabled (unlock_at_station_level = -1) pending
+# the balancing pass. The capital hulls are the exception: battleship is gated
+# to station level 1, and dreadnought (level 2) requires battleship to be
+# unlocked first (unlock_requires) — a demonstration of the prerequisite chain.
+#
 # Size classes:
 #   xs  drone                      1 cell — exactly one 1x1 module
 #   s   frigate, destroyer         no 2x2 area anywhere: only 1x1/1x2/1x3/L modules fit
@@ -169,7 +174,7 @@ sensor_range_m = 260
 # so no 2x6 drone hangar fits.
 [[ship]]
 id = "battleship"
-unlock_at_station_level = -1
+unlock_at_station_level = 1
 layout = [
     "XOOOOX",
     "OOOOOO",
@@ -203,7 +208,8 @@ sensor_range_m = 280
 # stay the only hangar hull. Bow and stern strips hold supports.
 [[ship]]
 id = "dreadnought"
-unlock_at_station_level = -1
+unlock_at_station_level = 2
+unlock_requires = ["battleship"]
 layout = [
     "XXXOOOOOXXX",
     "OOOXOOOXOOO",