Add Ground Zero map boundary

Docs/TechnicalDesignDocument.md

@@ -173,6 +173,13 @@ rechecks the placed `AGR_DemoPlayerStart` against the same constraints so future
 map, resource, water, or terrain changes cannot silently move the player below
 sea level, into steep terrain, into water, or into a dense resource cluster.
 
+The Ground Zero map also contains `AGR_GroundZeroMapBoundary`, a native
+`AAgrarianMapBoundaryVolume` centered on the 1 km x 1 km MVP tile. For the MVP,
+the boundary clamps server-authoritative player pawns back inside the loaded
+tile with a small padding rather than allowing players to walk into missing
+neighbor terrain. The actor exposes a warning distance hook so later UI can
+present an in-world or HUD notice before a clamp occurs.
+
 First-pass sky and lighting use `AAgrarianSkyLightingController`. The controller
 owns movable sun, skylight, and exponential-height-fog components and reads the
 replicated `AAgrarianGameState` time, active tile sunrise/sunset, weather state,