feat(writers): stamp RelSpec version in generated headers; preserve DBML column order

- Add pkg/buildinfo with Version/BuildDate (set via ldflags, VCS fallback);
  cmd/relspec now sources version info from it
- Emit "RelSpec <version> (built: <date>)" in generated file headers for
  bun, gorm, drizzle, pgsql (incl. migration), mssql, sqlite writers
- pgsql writer: getSortedColumns now sorts by Sequence then Name so the
  streaming WriteSchema/WriteDatabase path preserves source column order
- dbml reader: mergeTable re-bases merged-in column Sequence values past the
  existing max, fixing colliding sequences (and alphabetical fallback) when a
  table is split across multiple DBML files
- Tests for bun/gorm header + column order, and dbml multi-file merge ordering

pkg/readers/dbml/reader_test.go

@@ -652,6 +652,22 @@ func TestReadDirectory_TableMerging(t *testing.T) {
 	if emailCol.Type != "varchar(255)" {
 		t.Errorf("Expected email type 'varchar(255)', got '%s'", emailCol.Type)
 	}
+
+	// Merged columns must keep declaration order (file 1: id, email; file 3:
+	// name, created_at) via strictly increasing, non-colliding Sequence values
+	// so downstream writers do not fall back to alphabetical order.
+	order := []string{"id", "email", "name", "created_at"}
+	var prev uint
+	for i, name := range order {
+		col := usersTable.Columns[name]
+		if col.Sequence == 0 {
+			t.Fatalf("column %q has zero Sequence after merge", name)
+		}
+		if i > 0 && col.Sequence <= prev {
+			t.Errorf("column %q Sequence %d not greater than previous %d (order not preserved across files)", name, col.Sequence, prev)
+		}
+		prev = col.Sequence
+	}
 }
 
 func TestReadDirectory_CommentedRefsLast(t *testing.T) {