Restructure balancing docs into docs/balancing/

bin/app/data/config/world.toml

@@ -19,6 +19,8 @@ enemy_buffer_width_tiles = 20
 
 [expansion]
 columns_per_expansion_tiles = 10
+# x = expansions already purchased; ~1 per cycle mid-game, decelerating
+# to 2-3 cycles late (docs/balancing/derived.md).
 cost_building_blocks_formula = "300 + 50*x + 10*x*x"
 
 [push]
@@ -35,7 +37,7 @@ artifact_chance_formula = "0.05 * x"  # 5% chance per station level
 artifact_win_count      = 5
 
 [waves]
-# Tuned against the factory-size curve (content_design.md, balancing
+# Tuned against the factory-size curve (docs/balancing/targets.md, balancing
 # targets): stays below the player's achievable military output early,
 # crosses it around the late boundary (~cycle 15), overwhelms by ~24.
 threat_rate_formula = "2*x + 0.15*x*x"