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/recipes.toml

@@ -1,7 +1,7 @@
 # recipes.toml
 #
-# Production tree v2 (see docs/content_design.md, "Production tree v2" and
-# "Numbers — first pass"). Quantities and durations are tuned so that every
+# Production tree v2 (structure in docs/content_design.md, numbers with
+# derivations in docs/balancing/derived.md). Quantities and durations are tuned so that every
 # fitted ship lands on the threat-cost ladder and the ratio curve is
 # realized: tier 1 ratios are 1:1, tier 2 ratios are 2:3, tier 3+ ratios
 # are deliberately strange.
@@ -14,7 +14,7 @@
 #                                   rock IS ore)
 #   mid    + quartz                 geode deposits in expansion territory
 #                                   (deposit gating pending — see action
-#                                   item 5 in docs/progression_design.md;
+#                                   item 5 in docs/balancing/README.md;
 #                                   until then quartz mines anywhere)
 #   late   + voidsteel              battle-forged: ONLY from reprocessing
 #                                   salvaged scrap, so capital production
@@ -141,7 +141,7 @@ outputs = [{item = "copper_coil", amount = 1}]
 duration_seconds = 1.5
 
 # Depth-3 chain (ore -> ingot -> plate -> block) is the factory's
-# doubling-time knob; see the block economy rules in progression_design.md.
+# doubling-time knob; see the block economy rules in docs/balancing/rules.md.
 # Explicitly unlocked at start (-1): building blocks appear in no
 # schematic's materials, so implicit unlocking can never reach this recipe.
 [[recipe]]