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/README.md

@@ -137,6 +137,41 @@ indexes {
 }
 ```
 
+### Dialect directives
+
+Dialect directives stored on a model object's `Metadata` (namespace `postgres`,
+`sqlite`, …) are re-emitted verbatim, one line per directive, at the location
+they belong to:
+
+```dbml
+@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)
+  @sqlite: without rowid
+
+  indexes {
+    (created_at) [name: 'idx_events_created']
+    @postgres: with (fillfactor=90)
+  }
+}
+```
+
+| Emitted at | From |
+|------------|------|
+| Before the first table | `Database.Metadata` |
+| After a column line, as `@ns(col): …` | `Column.Metadata` |
+| After an index line, inside `indexes { }` | `Index.Metadata` |
+| After the `indexes` block, before `Note:` | `Table.Metadata` |
+
+Output is deterministic (ordered by namespace, then source line, then args), so a
+`DBML → model → DBML` round-trip is idempotent. See
+[`docs/DBML_DIRECTIVES.md`](../../../docs/DBML_DIRECTIVES.md) for the grammar and
+the list of directives the PostgreSQL and SQLite writers translate to SQL.
+
 ## Type Mapping
 
 | SQL Type | DBML Type |