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/gorm/template_data.go

@@ -4,6 +4,7 @@ import (
 	"sort"
 	"strings"
 
+	"git.warky.dev/wdevs/relspecgo/pkg/buildinfo"
 	"git.warky.dev/wdevs/relspecgo/pkg/models"
 	"git.warky.dev/wdevs/relspecgo/pkg/writers"
 )
@@ -11,6 +12,7 @@ import (
 // TemplateData represents the data passed to the template for code generation
 type TemplateData struct {
 	PackageName string
+	GeneratedBy string
 	Imports     []string
 	Models      []*ModelData
 	Config      *MethodConfig
@@ -79,6 +81,7 @@ func NewTemplateData(packageName string, config *MethodConfig) *TemplateData {
 
 	return &TemplateData{
 		PackageName: packageName,
+		GeneratedBy: buildinfo.GeneratedComment(),
 		Imports:     make([]string, 0),
 		Models:      make([]*ModelData, 0),
 		Config:      config,