docs: establish official design documentation

docs/SHIPS.md

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+# Ships
+
+This document defines the intended role of ships in the simulation.
+
+Ships are mobile actors that operate across the spatial model in [SPACES.md](/home/jbourdon/repos/space-game/docs/SPACES.md) under the authority of commanders defined in [COMMANDERS.md](/home/jbourdon/repos/space-game/docs/COMMANDERS.md).
+
+## Core Principles
+
+- every active ship should have a commander
+- ships operate in explicit movement regimes
+- ships execute logistics, combat, construction, scouting, and transport roles
+- ship capability depends on hull, modules, and commander intent
+
+## Ship Roles
+
+Common roles include:
+
+- mining
+- hauling
+- construction
+- escort
+- patrol
+- combat
+- scouting
+- passenger or utility support later
+
+From a simulation point of view, the important distinction is not only hull category but also commander role:
+
+- self-directed industrial ship
+- station-assigned ship
+- fleet-assigned ship
+- faction-directed strategic ship
+
+## Movement
+
+Ships should move according to the layered spatial model:
+
+- thrusters in `local-space`
+- warp in `system-space`
+- stargate or FTL for inter-system travel
+
+Ships should not behave as if an entire system is one continuous local dogfighting field.
+
+## Command Requirements
+
+A ship without a commander should be severely limited.
+
+At minimum, it should not perform meaningful autonomous loops such as:
+
+- mining cycles
+- trade runs
+- route planning
+- combat patrols
+- logistics procurement
+
+Ship commanders should be able to receive work from:
+
+- player direct orders
+- a superior commander
+- economic opportunities exposed by the market
+
+Some ships should also specialize in worker transport, provided they carry the required human habitat module defined in [WORKFORCE.md](/home/jbourdon/repos/space-game/docs/WORKFORCE.md).
+
+Ship work selection should also obey policy restrictions such as allowed trade partners, regional limits, and station-serving constraints. See [POLICIES.md](/home/jbourdon/repos/space-game/docs/POLICIES.md).
+
+## Capabilities
+
+Ship capability should depend on:
+
+- hull class
+- fitted modules
+- cargo and storage compatibility
+- propulsion capability
+- docking compatibility
+- optional warp and FTL capability
+
+Cargo and movement capability should materially affect which market opportunities a ship can accept.
+
+This should eventually include item-class compatibility as described in [ITEMS.md](/home/jbourdon/repos/space-game/docs/ITEMS.md).
+
+Those capabilities should primarily come from fitted modules as described in [MODULES.md](/home/jbourdon/repos/space-game/docs/MODULES.md).
+
+## Relationship To Other Documents
+
+- [SPACES.md](/home/jbourdon/repos/space-game/docs/SPACES.md)
+- [COMMANDERS.md](/home/jbourdon/repos/space-game/docs/COMMANDERS.md)
+- [ECONOMY.md](/home/jbourdon/repos/space-game/docs/ECONOMY.md)
+- [STATIONS.md](/home/jbourdon/repos/space-game/docs/STATIONS.md)
+- [WORKFORCE.md](/home/jbourdon/repos/space-game/docs/WORKFORCE.md)
+- [ITEMS.md](/home/jbourdon/repos/space-game/docs/ITEMS.md)
+
+- [MODULES.md](/home/jbourdon/repos/space-game/docs/MODULES.md)
+- [POLICIES.md](/home/jbourdon/repos/space-game/docs/POLICIES.md)