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.go

@@ -0,0 +1,23 @@
+package dbml
+
+import (
+	"git.warky.dev/wdevs/relspecgo/pkg/models"
+)
+
+// directiveLines renders every dialect directive stored in meta back to its DBML
+// source form, one line per directive, each prefixed with indent. When target is
+// non-empty it is emitted as the "(column)" target, e.g.
+// "  @postgres(id): identity always". Order is deterministic (see
+// models.GetDirectives).
+func directiveLines(meta map[string]any, indent, target string) []string {
+	directives := models.GetDirectives(meta)
+	if len(directives) == 0 {
+		return nil
+	}
+
+	lines := make([]string, 0, len(directives))
+	for _, d := range directives {
+		lines = append(lines, indent+models.FormatDirectiveLine(d, target))
+	}
+	return lines
+}