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

Makefile

@@ -22,7 +22,7 @@ GOVULNCHECK   = go run golang.org/x/vuln/cmd/govulncheck@latest
 # Version information
 VERSION := $(shell git describe --tags --always --dirty 2>/dev/null || echo "dev")
 BUILD_DATE := $(shell date -u +"%Y-%m-%d %H:%M:%S UTC")
-LDFLAGS := -X 'main.version=$(VERSION)' -X 'main.buildDate=$(BUILD_DATE)'
+LDFLAGS := -X 'git.warky.dev/wdevs/relspecgo/pkg/buildinfo.Version=$(VERSION)' -X 'git.warky.dev/wdevs/relspecgo/pkg/buildinfo.BuildDate=$(BUILD_DATE)'
 
 # Auto-detect container runtime (Docker or Podman)
 CONTAINER_RUNTIME := $(shell \