Replaces the hardcoded seed=0 with the seed received in StartupMessage,
threading it through SimulationPlugin -> EconomyPlugin / SimRng. Integration
test fixtures updated for the new SimulationPlugin { seed } signature.
validate_movement now checks both terrain walkability AND tile
occupancy. Builds a spatial index of occupied tiles from stationary
entities, then resolves movers in order — first valid claim wins.
Same spatial pattern needed for D-026 simulation tiers (30-80 active
NPCs) and future pathfinding occupied-tile awareness.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
TilePosition component with discrete grid coordinates, flat-storage
WalkabilityMap resource with O(1) can_move_to() lookup, MoveIntent
component and validate_movement system. Movement validated against
walkability map each tick, blocking all NPC and player movement
through unwalkable tiles. 11 unit tests + 1 integration test.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
jpmschweitzer
