FK columns can be directly referenced from now on

common/proto/table_definition.proto

@@ -62,7 +62,9 @@ message PostTableDefinitionRequest {
   // two are named side by side. The "_id" suffix is allowed.
   string table_name = 1;
 
-  // The table's columns, including its links. System columns are added on top.
+  // The table's columns, including its links and stored linked projections.
+  // A projection uses field_type FROM(link_column.source_column); its physical
+  // type and money metadata are inferred from the linked source column.
   repeated ColumnDefinition columns = 3;
 
   // Column names to index, matching names declared above. System columns
@@ -139,6 +141,7 @@ message AddTableColumnsRequest {
   string table_name = 2;
 
   // New user-defined columns only. Existing columns cannot be changed here.
+  // Stored linked projections use field_type FROM(link_column.source_column).
   repeated ColumnDefinition columns = 3;
 
   // Optional indexes for the new columns only.
@@ -188,6 +191,9 @@ message ColumnDefinition {
   //   LINK(table) → BIGINT referencing that table in the same profile, indexed
   //                 automatically. A table may hold several links to the same
   //                 target as long as the columns are named differently.
+  //   FROM(link.source) → stored, read-only copy of an atomic column reached
+  //                 through another column in this definition whose type is
+  //                 LINK(...). SQL type and money metadata are inferred.
   // DECIMAL args must be integers (no sign, no dot, no leading zeros);
   // s ≤ p and p ≥ 1.
   string field_type = 2;