Scenarios Cut 1: RunStart stages a new run; the Scenario catalog type

RunStart (ServerShared, no Unity type) owns what a new run starts with
(docs/scenarios-cut.md, 2.2):
- GenerateArena: the galaxy's entrance pack, generated under the scenario's
  Ambient rule. ZoneGenerator.GenerateZone gains `ambient`; false keeps
  planets, orbits and stations and generates no ship, turret or wanderer.
- Check: every failure staging a scenario would hit, found with no zone,
  touching no store.
- Stage: all-or-nothing. With no scenario, the generated starting-hull ship
  at the origin, as StartGame built it. With one, the player preset and the
  entities at their authored place and heading with cargo, stance set both
  ways through SetIff, a Minion for each piloted one; all through Zone.Admit.

Scenario (aetheria.scenario v1) joins AetheriaStores.CatalogTypes.
RunStartTests stage into a real prelude arena from the shipped catalog and
authored settings.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Assets/Scripts/ServerShared/LoadoutGenerator.cs

@@ -149,8 +149,8 @@ public class LoadoutGenerator
 
     // No galaxy means no availability to filter by: every product is on offer. A fixture generates loadouts that
     // way, so a test can exercise placement and products without standing up a whole galaxy; no game path does.
-    // Loadouts.Materialize takes availability as a predicate; a game spawner materializing presets is to pass this, so
-    // presets and generation share one availability rule. No game path materializes a preset yet.
+    // Loadouts.Materialize takes availability as a predicate; RunStart, the game's one preset spawner, passes this, so
+    // presets and generation share one availability rule.
     public bool IsAvailable(FactionProductData product) =>
         Galaxy == null || Galaxy.IsPrelude ||
         Galaxy.ContainsFaction(product.Manufacturer) && (Faction == null || Faction.Allegiance.ContainsKey(product.Manufacturer));