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/sqlite/writer.go

@@ -10,6 +10,7 @@ import (
 
 	_ "modernc.org/sqlite" // SQLite driver
 
+	"git.warky.dev/wdevs/relspecgo/pkg/buildinfo"
 	"git.warky.dev/wdevs/relspecgo/pkg/models"
 	"git.warky.dev/wdevs/relspecgo/pkg/writers"
 )
@@ -72,7 +73,7 @@ func (w *Writer) writeContent(db *models.Database) error {
 	// Write header comment
 	fmt.Fprintf(w.writer, "-- SQLite Database Schema\n")
 	fmt.Fprintf(w.writer, "-- Database: %s\n", db.Name)
-	fmt.Fprintf(w.writer, "-- Generated by RelSpec\n")
+	fmt.Fprintf(w.writer, "-- Generated by %s\n", buildinfo.GeneratedComment())
 	fmt.Fprintf(w.writer, "-- Note: SQLite has no schema concept; non-default schema names are flattened into table name prefixes (e.g., auth.sessions -> auth_sessions)\n\n")
 
 	// Enable foreign keys