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.

pkg/merge/merge_test.go

@@ -721,3 +721,32 @@ func TestComplexMerge(t *testing.T) {
 		t.Error("Expected ukey_users_guid constraint to exist")
 	}
 }
+
+func TestMergeDatabases_Metadata(t *testing.T) {
+	target := &models.Database{Metadata: map[string]any{
+		"refs": []string{"a", "b"},
+		"name": "target",
+	}}
+	source := &models.Database{Metadata: map[string]any{
+		"refs":  []string{"b", "c"},
+		"name":  "source",
+		"extra": []string{"x"},
+	}}
+
+	MergeDatabases(target, source, nil)
+
+	if got := target.Metadata["refs"].([]string); strings.Join(got, ",") != "a,b,c" {
+		t.Errorf("refs = %v, want [a b c]", got)
+	}
+	if got := target.Metadata["name"]; got != "target" {
+		t.Errorf("name = %v, want target (existing scalar keys are kept)", got)
+	}
+	extra := target.Metadata["extra"].([]string)
+	if strings.Join(extra, ",") != "x" {
+		t.Errorf("extra = %v, want [x]", extra)
+	}
+	source.Metadata["extra"].([]string)[0] = "changed"
+	if extra[0] != "x" {
+		t.Error("copied slice must not alias the source")
+	}
+}