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/sqlite/template_functions.go

@@ -22,9 +22,10 @@ func GetTemplateFuncs(opts *writers.WriterOptions) template.FuncMap {
 		"format_constraint_name": func(schema, table, constraint string) string {
 			return FormatConstraintName(schema, table, constraint, opts)
 		},
-		"join":  strings.Join,
-		"lower": strings.ToLower,
-		"upper": strings.ToUpper,
+		"join":           strings.Join,
+		"lower":          strings.ToLower,
+		"upper":          strings.ToUpper,
+		"column_collate": sqliteColumnCollate,
 	}
 }