feat(dbml): @postgres/@sqlite dialect directives (#19)

Add parseable `@<namespace>[(<target>)]: <args>` directives embedded in DBML.
They are stored losslessly on each object's Metadata, round-trip unchanged
through the DBML writer, and are translated to SQL only by the writer for the
matching dialect.

- models: Directive type + catalog; Metadata map added to Column and Index
- dbml reader: parse and attach directives at database/table/column/index
  level; line-numbered errors; repeatable by default with singleton duplicate
  detection. Fixes a preexisting bug where an `indexes {}` closing brace ended
  the table early, dropping trailing Note: and directive lines.
- dbml writer: re-emit directives at their location; idempotent output
- pgsql writer: PARTITION BY / INHERITS / WITH / TABLESPACE (table),
  STORAGE / COMPRESSION / identity (column), WITH / TABLESPACE (index)
- sqlite writer: WITHOUT ROWID / STRICT (table), COLLATE (column)
- --strict-directives flag on ReaderOptions and WriterOptions
- docs/DBML_DIRECTIVES.md + reader/writer READMEs

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ss2MY5J11cRGwEz86ZXk7d

pkg/writers/dbml/directives_test.go

@@ -0,0 +1,97 @@
+package dbml
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	dbmlreader "git.warky.dev/wdevs/relspecgo/pkg/readers/dbml"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"git.warky.dev/wdevs/relspecgo/pkg/models"
+	"git.warky.dev/wdevs/relspecgo/pkg/readers"
+	"git.warky.dev/wdevs/relspecgo/pkg/writers"
+)
+
+const directiveSrc = `@postgres: search_path myapp
+
+Table myapp.events {
+  id bigint [pk]
+  created_at timestamp [not null]
+  @postgres(id): identity always
+  @postgres: partition by RANGE (created_at)
+  @postgres: tablespace fast_data
+  @sqlite: without rowid
+
+  indexes {
+    (created_at) [name: 'idx_events_created']
+    @postgres: with (fillfactor=90)
+  }
+}
+`
+
+func writeDBML(t *testing.T, db *models.Database) string {
+	t.Helper()
+	out := filepath.Join(t.TempDir(), "out.dbml")
+	require.NoError(t, NewWriter(&writers.WriterOptions{OutputPath: out}).WriteDatabase(db))
+	b, err := os.ReadFile(out)
+	require.NoError(t, err)
+	return string(b)
+}
+
+func readDBML(t *testing.T, src string) *models.Database {
+	t.Helper()
+	f := filepath.Join(t.TempDir(), "in.dbml")
+	require.NoError(t, os.WriteFile(f, []byte(src), 0o644))
+	db, err := dbmlreader.NewReader(&readers.ReaderOptions{FilePath: f}).ReadDatabase()
+	require.NoError(t, err)
+	return db
+}
+
+func collectDirectives(db *models.Database) map[string][]string {
+	got := map[string][]string{}
+	add := func(loc string, meta map[string]any) {
+		for _, d := range models.GetDirectives(meta) {
+			got[loc] = append(got[loc], models.FormatDirectiveLine(d, ""))
+		}
+	}
+	add("database", db.Metadata)
+	for _, s := range db.Schemas {
+		for _, tbl := range s.Tables {
+			add("table:"+tbl.Name, tbl.Metadata)
+			for _, c := range tbl.Columns {
+				add("column:"+c.Name, c.Metadata)
+			}
+			for _, i := range tbl.Indexes {
+				add("index:"+i.Name, i.Metadata)
+			}
+		}
+	}
+	return got
+}
+
+func TestDirectives_RoundTrip(t *testing.T) {
+	db1 := readDBML(t, directiveSrc)
+	out1 := writeDBML(t, db1)
+	db2 := readDBML(t, out1)
+	out2 := writeDBML(t, db2)
+
+	assert.Equal(t, out1, out2, "DBML directive output should be idempotent")
+	assert.Equal(t, collectDirectives(db1), collectDirectives(db2), "directives preserved through round-trip")
+
+	// Spot-check each location survived.
+	d := collectDirectives(db2)
+	assert.Contains(t, d["database"], "@postgres: search_path myapp")
+	assert.Contains(t, d["table:events"], "@postgres: partition by RANGE (created_at)")
+	assert.Contains(t, d["table:events"], "@sqlite: without rowid")
+	assert.Contains(t, d["column:id"], "@postgres: identity always")
+	assert.Contains(t, d["index:idx_events_created"], "@postgres: with (fillfactor=90)")
+}
+
+func TestDirectives_WriterEmitsColumnTarget(t *testing.T) {
+	db := readDBML(t, directiveSrc)
+	out := writeDBML(t, db)
+	assert.Contains(t, out, "@postgres(id): identity always")
+}