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/writers/pgsql/migration_writer.go

@@ -7,6 +7,7 @@ import (
 	"sort"
 	"strings"
 
+	"git.warky.dev/wdevs/relspecgo/pkg/buildinfo"
 	"git.warky.dev/wdevs/relspecgo/pkg/models"
 	"git.warky.dev/wdevs/relspecgo/pkg/pgsql"
 	"git.warky.dev/wdevs/relspecgo/pkg/writers"
@@ -143,7 +144,7 @@ func (w *MigrationWriter) WriteMigration(model, current *models.Database) error
 
 	// Write header
 	fmt.Fprintf(w.writer, "-- PostgreSQL Migration Script\n")
-	fmt.Fprintf(w.writer, "-- Generated by RelSpec\n")
+	fmt.Fprintf(w.writer, "-- Generated by %s\n", buildinfo.GeneratedComment())
 	fmt.Fprintf(w.writer, "-- Source: %s -> %s\n", current.Name, model.Name)
 	if w.options.ContinueOnError {
 		fmt.Fprintf(w.writer, "\\set ON_ERROR_STOP off\n")