Pacing pass: unlock ladder, threat rate, economy values

Replace the everything-at--1 test setup with the real progression:
starting set is exactly drone/frigate/railgun_s/salvager; the ladder
runs destroyer+repair+armor at level 0, supports at 1, the quartz gate
(cruiser, railgun_m, afterburner, chip/hardened shortcuts) at 2,
battlecruiser at 4, battleship+railgun_l at 6, dreadnought at 8,
carrier+drone_hangar at 9, with unlock_requires edges cruiser ->
battlecruiser -> battleship -> dreadnought/carrier and railgun_m ->
railgun_l.

world.toml: threat_rate_formula 2*x + 0.15*x*x (below the player's
achievable military output early, crossing at the late boundary,
overwhelming by cycle ~24), starting_building_blocks 1000 -> 200,
expansion cost 200 -> 400 flat (placeholder until action item 4),
artifact_win_count 3 -> 5 (winning means choosing the artifact over a
schematic five times across ~7 cumulative offers).

Bug fix found during the pass: the building_block recipe carried no
unlock_at_station_level, and since building blocks appear in no
schematic's materials, implicit unlocking could never reach it - the
recipe was silently locked at game start. Now explicitly -1.

All values and derivations documented in content_design.md, "Pacing
pass - first values".

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DyCu8vwChKMbLJQ3xosYEN

bin/app/data/config/ships.toml

@@ -84,7 +84,7 @@ sensor_range_m = 200
 # mount medium hardware.
 [[ship]]
 id = "destroyer"
-unlock_at_station_level = -1
+unlock_at_station_level = 0
 layout = [
     "OXOXO",
     "OOOOO",
@@ -120,7 +120,7 @@ sensor_range_m = 220
 # supports; no 3x3 area exists for an l gun.
 [[ship]]
 id = "cruiser"
-unlock_at_station_level = -1
+unlock_at_station_level = 2
 layout = [
     "XOOX",
     "OOOO",
@@ -158,7 +158,8 @@ sensor_range_m = 250
 # stern leave no 3x3 area for an l gun and no 2x6 area for a drone hangar.
 [[ship]]
 id = "battlecruiser"
-unlock_at_station_level = -1
+unlock_at_station_level = 4
+unlock_requires = ["cruiser"]
 layout = [
     "OOXXOO",
     "OOOOOO",
@@ -200,7 +201,8 @@ sensor_range_m = 260
 # so no 2x6 drone hangar fits.
 [[ship]]
 id = "battleship"
-unlock_at_station_level = 1
+unlock_at_station_level = 6
+unlock_requires = ["battlecruiser"]
 layout = [
     "XOOOOX",
     "OOOOOO",
@@ -242,7 +244,7 @@ sensor_range_m = 280
 # stay the only hangar hull. Bow and stern strips hold supports.
 [[ship]]
 id = "dreadnought"
-unlock_at_station_level = 2
+unlock_at_station_level = 8
 unlock_requires = ["battleship"]
 layout = [
     "XXXOOOOOXXX",
@@ -286,7 +288,8 @@ sensor_range_m = 300
 # the lower decks hold supports and 2x2 point-defense m guns.
 [[ship]]
 id = "carrier"
-unlock_at_station_level = -1
+unlock_at_station_level = 9
+unlock_requires = ["battleship"]
 layout = [
     "XOOOOOOOOX",
     "OOOOOOOOOO",