design config

docs/content_design.md

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+# Content Design — Ships & Modules
+
+First real-content iteration (June 2026). This pass defines ship hull grids
+and module surface masks only. Stats, materials, recipes, and threat costs in
+the config files are placeholders; the recipe pass and the balancing pass
+come later.
+
+## Design principle: footprint gating
+
+Which module fits on which hull is controlled purely by geometry — no
+explicit allow-lists. Each hull grid is shaped so that it physically cannot
+contain the footprint of modules from a larger size class. This keeps the
+rules transparent to the player ("it doesn't fit because there is no room")
+and makes them trivially moddable through the config files alone.
+
+### Module footprint ladder
+
+| Footprint | Modules | Smallest hull that fits it |
+|-----------|---------|----------------------------|
+| 1x1 | laser_cannon_s, salvager, repair_tool | drone |
+| 1x2 | maneuvering_thrusters, sensor_booster, armor_plates | frigate |
+| 1x3 | afterburner | frigate (eats most of it) |
+| L-shape (3 cells) | weapon_stabilizer, weapon_primer, weapon_upgrade | frigate |
+| 2x2 | laser_cannon_m, drone_bay | cruiser |
+| 3x3 | laser_cannon_l | battleship |
+| 2x6 | drone_hangar | dreadnought / carrier |
+
+### Hull grids
+
+`O` = buildable cell, `X` = hull structure (not buildable).
+
+**drone (xs, 1 cell)** — exactly one 1x1 module: a small gun, a salvager, or
+a repair tool. This is what makes drone roles swappable.
+
+    O
+
+**frigate (s, 5 cells)** — plus shape. Every 1x2 placement crosses the center
+cell, so at most ONE 1x2 support fits; alternatively one L-shaped weapon
+modifier or one afterburner through the center line. Gun-boat with one or two
+support modules, as intended.
+
+    XOX
+    OOO
+    XOX
+
+**destroyer (s, 8 cells)** — gun deck with three turret bumps. More cells
+than the frigate (more small guns), but still no 2x2 area anywhere, so medium
+hardware can never be mounted.
+
+    OXOXO
+    OOOOO
+
+**cruiser (m, 12 cells)** — notched corners. Fits at most two 2x2 m guns
+(stacked through the middle), leaving the side cells for supports. No 3x3
+area.
+
+    XOOX
+    OOOO
+    OOOO
+    XOOX
+
+**battlecruiser (m, 16 cells)** — split bow with two gun cheeks, tapered
+stern. Fits three 2x2 m guns — one more than the cruiser — with small support
+slots left over. The bow split and stern taper prevent any 3x3 area (no l
+gun) and any 2x6 area (no drone hangar).
+
+    OOXXOO
+    OOOOOO
+    XOOOOX
+    XXOOXX
+
+**battleship (l, 20 cells)** — 5 wide, so at most ONE 3x3 l gun fits, and
+mounting it consumes nearly half the grid, leaving only narrow strips for
+supports. Without an l gun it flexibly fits several m guns. Too narrow for a
+drone hangar.
+
+    XOOOX
+    OOOOO
+    OOOOO
+    OOOOO
+    XOXOX
+
+**dreadnought (xl, 42 cells)** — 11-wide, 3-deep main battery deck: three 3x3
+l guns fit side by side with spacer columns to spare, plus bow/stern strips
+for supports. Wide enough for a drone hangar too, if the player wants to
+trade a gun battery for one.
+
+    XXXOOOOOXXX
+    OOOOOOOOOOO
+    OOOOOOOOOOO
+    OOOOOOOOOOO
+    XXOOXXXOOXX
+
+**carrier (xl, 37 cells)** — the top flight deck (rows 0–1) is the only
+region wide enough for the 2x6 drone hangar, and exactly one fits. The middle
+deck row is broken up by elevator shafts (X cells placed so every 3-column
+window hits one), which is what prevents any 3x3 l gun from ever fitting.
+Lower decks hold supports and 2x2 point-defense m guns.
+
+    XOOOOOOOOX
+    OOOOOOOOOO
+    OOXOOXOOXO
+    XOOOOOOOOX
+    XXXOOOOXXX
+
+### Verified gating matrix
+
+Checked programmatically against the configs (all four mask rotations,
+all placements):
+
+| Footprint | drone | frigate | destroyer | cruiser | battlecruiser | battleship | dreadnought | carrier |
+|-----------|:-:|:-:|:-:|:-:|:-:|:-:|:-:|:-:|
+| 1x1 | x | x | x | x | x | x | x | x |
+| 1x2 | | x | x | x | x | x | x | x |
+| 1x3 | | x | x | x | x | x | x | x |
+| L-shape | | x | x | x | x | x | x | x |
+| 2x2 | | | | x | x | x | x | x |
+| 3x3 | | | | | | x | x | x* |
+| 2x6 | | | | | | | x | x |
+
+\* dreadnought only; the carrier's elevator shafts block all 3x3 placements.
+Max disjoint l guns: battleship 1, dreadnought 3. Max disjoint m guns:
+cruiser 2, battlecruiser 3.
+
+## Deliberate placeholders / open questions for later passes
+
+- All new hulls have `threat.cost_formula = "0"` so enemy waves do not spawn
+  them yet (WaveSystem treats any ship with positive threat cost as wave-
+  eligible, regardless of unlock level). The balancing pass should set real
+  threat costs together with `default_modules` loadouts so waves spawn them
+  armed.
+- All new hulls are `unlock_at_station_level = -1` (available from the start)
+  to make layout testing easy; the progression pass should stagger these.
+- Ship hull material items (`frigate_hull` … `carrier_hull`) and the new
+  module items (`laser_cannon_m_module`, `laser_cannon_l_module`,
+  `drone_bay_module`, `drone_hangar_module`, …) have no recipes yet — that is
+  the recipe pass. The old `laser_cannon_xs_module` recipe is orphaned (the
+  module was renamed to `laser_cannon_s`, consuming `laser_cannon_s_module`,
+  which already has a recipe).
+- `drone_bay` and `drone_hangar` are footprint-only placeholders: the drone
+  launching capability does not exist in the simulation yet, so they define
+  no capability section.
+- Renames in this pass: `laser_cannon_xs` → `laser_cannon_s` (the old 2x2
+  `laser_cannon_s` became `laser_cannon_m`), `armor_plate` → `armor_plates`,
+  `manuvering_thrusters` → `maneuvering_thrusters` (typo fix). Test data
+  under `bin/test/data/config` intentionally still uses the old ids — it is
+  an independent fixture set.