Document safe Page-list fallbacks

Server-and-Operator-Guide.md

@@ -26,7 +26,7 @@ Alternatively, an allowlist can limit a particular Page family. For example, `al
 
 This is a *future-generation policy*, not a ban on player-owned Pages. It does not remove items from Creative or JEI, reject a hand-authored Descriptive Book, or rewrite an existing Age. New random Ages save their list policy when bound. Libraries apply the current lists when they generate; existing contents are unchanged. Native Minecraft biome presets are not assembled from individual biome Pages, so blocking a biome Page does not remove that biome from a Native distribution.
 
-Keep at least one eligible choice in each required family (such as terrain, lighting, weather, and biome distribution). If a list removes every required choice, the Binder reports that the random Page lists have no available choices instead of generating an invalid Age.
+If a required random family has no eligible choices, Age generation uses a stable fallback instead of failing: Standard terrain (or the first loaded profile if Standard is unavailable), Native biome distribution, Normal lighting and weather, No Seasons, or an inherited sky. If individual biome Pages leave too few biomes for the chosen distribution, the pool is filled from the terrain's normal biome list, preferring Plains on surface terrain and Deep Dark in caves. These are safety exceptions to the lists, not extra Pages added to library loot. Optional features and celestial bodies simply disappear when every choice in their family is blocked.
 
 ## Operator commands