Restructure balancing docs into docs/balancing/

Split the cluttered progression_design.md and content_design.md into
separated documents by role:

- docs/balancing/rules.md    - design rules and principles (moved from
                               progression_design.md, which is removed)
- docs/balancing/targets.md  - the chosen base numbers: run shape,
                               factory curve, threat ladder (achieved
                               values adopted), combat anchors, pacing
                               anchors
- docs/balancing/derived.md  - current tuned state of all derived
                               numbers, mirroring the configs
- docs/balancing/process.md  - pass order, tuning discipline learned in
                               arena rounds 1-5, tools, next-round
                               checklist
- docs/balancing/history.md  - chronological record: decisions, numbers
                               pass, calculator bugs, arena rounds,
                               pacing pass
- docs/balancing/README.md   - index, status, open action items (moved
                               from progression_design.md)

content_design.md slims back down to actual content: footprint gating,
hull grids, gating matrix (module names updated to railguns), and the
production tree structure with its fiction - numbers, anchors, arena
logs, and pacing all moved to docs/balancing/. Stale references in the
config comments updated to the new locations.

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

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"