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/writer.go

@@ -86,6 +86,10 @@ type WriterOptions struct {
 	// Prisma7 enables Prisma 7-specific output for Prisma writers.
 	Prisma7 bool
 
+	// StrictDirectives makes dialect directive translation fail on an
+	// unsupported key for the writer's own namespace instead of skipping it.
+	StrictDirectives bool
+
 	// ContinueOnError instructs SQL writers to prepend `\set ON_ERROR_STOP off`
 	// to their output so that psql continues past errors instead of stopping.
 	ContinueOnError bool