refactor AI system

docs/requirements.md

@@ -161,13 +161,16 @@ Modules in `modules.toml` define a `surface_mask` — a list of strings that des
   - Stance: aggressive (advance toward enemies) / defensive (hold position near asteroid).
   - Target priority: closest / highest HP / structures first.
 - REQ-SHP-RALLY: After spawning, aggressive-stance ships with weapon modules move to and loiter at the **rally point** — the midpoint between the two player defence stations (center of their Y-span, at the player defence stations' X position). While at the rally point, ships still engage any enemy that enters sensor range. Every `world.toml [world].departure_interval_seconds` seconds (default 20), all ships with weapon modules currently at the rally point depart simultaneously and begin their normal aggressive advance toward the enemy. The departure timer is global and shared across all shipyards; it is not reset by individual ship arrivals at the rally point.
-- REQ-SHP-SALVAGE: Ships with at least one **salvage module** (player) — patrol by moving forward (rightward, away from the asteroid) while searching sensor range. If scrap enters sensor range, move to it; when it is within a module's `collection_range`, that module collects it (consuming the scrap entity). Once all cargo is full, fly to a Salvage Bay and deliver; after delivery, resume patrol. If an enemy ship enters sensor range while not currently targeting or carrying scrap, turn back (move toward the asteroid) until the enemy is no longer in sensor range, then resume patrol. Ships with salvage modules are vulnerable to enemy ships while operating.
+- REQ-SHP-SALVAGE: Ships with at least one **salvage module** (player) — patrol by moving forward (rightward, away from the asteroid) while searching sensor range. If scrap enters sensor range, move to it; when it is within a module's `collection_range`, that module collects it (consuming the scrap entity). Once all cargo is full, fly to a Salvage Bay and deliver; after delivery, resume patrol. If an enemy ship enters sensor range, the ship retreats (REQ-SHP-RETREAT) until no enemy is in sensor range, then resumes patrol — this applies regardless of whether the ship is targeting or carrying scrap. Ships with salvage modules are vulnerable to enemy ships while operating.
 
   Each salvage module instance operates independently: it has its own cargo hold (`cargo_capacity`), collection range (`collection_range`), and collection rate (`collection_rate`, in collections per second). After collecting a piece of scrap, the module cannot collect again until `1 / collection_rate` seconds have elapsed. A ship with multiple salvage modules can therefore collect multiple pieces of scrap per tick (one per ready module), and installs of different module types may have different ranges and rates. The ship navigates based on the maximum collection range across all installed salvage modules.
-- REQ-SHP-REPAIR: Ships with at least one **repair module** (player) — patrol by moving forward (rightward, away from the asteroid) while searching sensor range. If a damaged player defence station or player ship enters sensor range, move to it and repair. If an enemy ship enters sensor range while not currently repairing, turn back (move toward the asteroid) until the enemy is no longer in sensor range, then resume patrol. The player can configure the target priority per shipyard:
+
+  Salvage collection and delivery are world-state changes performed every tick regardless of which behavior the ship is currently executing; the salvage behavior only governs where the ship navigates (toward scrap, toward a Salvage Bay, or — when retreating — toward the rally point).
+- REQ-SHP-REPAIR: Ships with at least one **repair module** (player) — patrol by moving forward (rightward, away from the asteroid) while searching sensor range. If a damaged player defence station or player ship enters sensor range, move to it and repair. If an enemy ship enters sensor range, the ship retreats (REQ-SHP-RETREAT) until no enemy is in sensor range, then resumes patrol. The player can configure the target priority per shipyard:
   - Defence stations first / ships first / nearest target.
 
-  Each repair module instance operates independently: it has its own repair rate (`repair_rate`) and repair range (`repair_range`). On each tick, a module first attempts to heal the ship's current behavior-level navigation target if that target is within the module's `repair_range` and is damaged (HP above zero and below maximum HP). If those conditions are not met — because the target is out of the module's `repair_range`, already at full health, or destroyed — the module independently searches for the nearest damaged friendly (player ship or player defence station) within its own `repair_range` and heals that instead. If no valid target is found within range, the module idles. A ship with multiple repair modules can therefore heal different targets simultaneously. Navigation is driven solely by the behavior-level target; individual module fallback targets do not affect which direction the ship moves.
+  Each repair module instance operates independently: it has its own repair rate (`repair_rate`) and repair range (`repair_range`). On each tick, a module first attempts to heal the ship's current behavior-level navigation target if that target is within the module's `repair_range` and is damaged (HP above zero and below maximum HP). If those conditions are not met — because the target is out of the module's `repair_range`, already at full health, or destroyed — the module independently searches for the nearest damaged friendly (player ship or player defence station) within its own `repair_range` and heals that instead. If no valid target is found within range, the module idles. A ship with multiple repair modules can therefore heal different targets simultaneously. Navigation is driven solely by the behavior-level target; individual module fallback targets do not affect which direction the ship moves. Repair healing is a world-state change applied every tick regardless of which behavior the ship is currently executing.
+- REQ-SHP-RETREAT: **Player ships retreat to the rally point (REQ-SHP-RALLY) when threatened.** A ship retreats while either condition holds: (a) its HP is below a low-HP threshold (currently 30% of its maximum HP); or (b) it has no weapon modules and an enemy ship is within its sensor range. Retreating takes priority over the ship's other behaviors and moves it toward the rally point; the ship resumes its normal behavior once neither condition holds. Enemy ships never retreat (REQ-SHP-ENEMY-AI).
 - REQ-SHP-ENEMY-AI: **Enemy ships** — engage the closest valid target (player defence station, HQ, or player ship) within their sensor range. If no target is in sensor range, they move toward the asteroid (leftward in world coordinates).
 - REQ-SHP-SCHEMATICS: The player selects a schematic per shipyard by clicking it. New schematics are unlocked by destroying enemy defence station sets (REQ-DEF-SCHEMATIC-DROP) — there is no physical loot to collect.