Function IsConversationHiddenByFieldSecurity

Deterministic Field-Level Security (FLS) gate for anchored conversations.

Returns true when the conversation must be hidden from user because:

1. entityInfo.FieldLevelSecurityApplies is true, AND
2. Either:
   • anchorType === 'Field' and anchorKey is in entityInfo.GetDeniedReadFields(user), OR
   • anchorMetadata.fieldName is in entityInfo.GetDeniedReadFields(user), OR
   • Any field listed in anchorMetadata.restrictedFields is in entityInfo.GetDeniedReadFields(user).

Runs in O(1) memory with zero database or LLM calls on the read path.