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/readers/dbml/README.md

@@ -93,6 +93,50 @@ Ref: posts.user_id > users.id [delete: cascade]
 - Indexes and composite indexes
 - Table notes and column notes
 - Enums
+- Dialect directives (`@postgres:` / `@sqlite:` — see below)
+
+## Dialect directives
+
+Lines of the form `@<namespace>[(<column>)]: <args>` embed database-specific
+features that plain DBML cannot express (partitioning, `WITHOUT ROWID`,
+tablespaces, index storage parameters, …). They are stored losslessly on the
+relevant object's `Metadata` and round-trip unchanged through the DBML writer;
+the PostgreSQL and SQLite writers translate the ones they understand to SQL.
+
+```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)
+  }
+}
+```
+
+| Position | Attaches to |
+|----------|-------------|
+| Before the first `Table {` | database |
+| Table body, no `(target)` | that table |
+| Table body, `(col)` target | column `col` (error if unknown) |
+| Inside `indexes { }` | the most recently listed index entry |
+
+`args` is preserved verbatim; the **key** (lowercased first token) drives
+duplicate detection. Repeated directives are kept in order; catalog "singleton"
+keys error on a second occurrence at the same location. All errors are
+line-numbered.
+
+`ReaderOptions.StrictDirectives` (CLI `--strict-directives`) turns an unknown
+namespace or key into an error instead of preserving it silently.
+
+See [`docs/DBML_DIRECTIVES.md`](../../../docs/DBML_DIRECTIVES.md) for the full
+grammar and the supported-directive matrix.
 
 ## Notes