fix(dbml): resolve commented cross-file // Ref: lines

Commented refs are collected per file and resolved against the combined
model after all inputs are loaded (directory, --from-list, merge, jobs).
Matched refs become FKs and relationships; duplicates of existing FKs are
skipped; missing targets are skipped with a warning; column type
mismatches warn.

Also keep reused index names within a DBML table instead of overwriting,
give a second FK to the same table a distinct relationship name, and make
the pgsql writer match relationships to FKs by name first.

cmd/relspec/merge.go

@@ -248,6 +248,7 @@ func runMerge(cmd *cobra.Command, args []string) error {
 	}
 
 	result := merge.MergeDatabases(targetDB, sourceDB, opts)
+	finalizeCommentedRefs(targetDB, stderrWarn)
 
 	// Update timestamp
 	targetDB.UpdateDate()