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/writer.go

@@ -72,6 +72,14 @@ func (w *Writer) databaseToDBML(d *models.Database) string {
 		sb.WriteString("\n")
 	}
 
+	if dirLines := directiveLines(d.Metadata, "", ""); len(dirLines) > 0 {
+		for _, line := range dirLines {
+			sb.WriteString(line)
+			sb.WriteString("\n")
+		}
+		sb.WriteString("\n")
+	}
+
 	for _, schema := range d.Schemas {
 		sb.WriteString(w.schemaToDBML(schema))
 	}
@@ -146,6 +154,11 @@ func (w *Writer) tableToDBML(t *models.Table) string {
 			fmt.Fprintf(&sb, " // %s", column.Comment)
 		}
 		sb.WriteString("\n")
+
+		for _, line := range directiveLines(column.Metadata, "  ", column.Name) {
+			sb.WriteString(line)
+			sb.WriteString("\n")
+		}
 	}
 
 	if len(t.Indexes) > 0 {
@@ -167,10 +180,20 @@ func (w *Writer) tableToDBML(t *models.Table) string {
 				fmt.Fprintf(&sb, " [%s]", strings.Join(indexAttrs, ", "))
 			}
 			sb.WriteString("\n")
+
+			for _, line := range directiveLines(index.Metadata, "    ", "") {
+				sb.WriteString(line)
+				sb.WriteString("\n")
+			}
 		}
 		sb.WriteString("  }\n")
 	}
 
+	for _, line := range directiveLines(t.Metadata, "  ", "") {
+		sb.WriteString(line)
+		sb.WriteString("\n")
+	}
+
 	note := strings.TrimSpace(t.Description + " " + t.Comment)
 	if note != "" {
 		fmt.Fprintf(&sb, "\n  Note: '%s'\n", note)