Merge pull request 'feat(dbml): @postgres/@sqlite dialect directives (#19)' (#27) from issue-19-dbml-directives into master

Reviewed-on: #27

docs/DBML_DIRECTIVES.md

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+# DBML Dialect Directives
+
+DBML has no dialect-neutral way to express database-specific features such as
+PostgreSQL table partitioning or SQLite `WITHOUT ROWID`. RelSpec adds **dialect
+directives** — explicit, parseable lines embedded in a `.dbml` file that are:
+
+- stored losslessly in the intermediate model (under each object's `Metadata`),
+- preserved unchanged through a `DBML → model → DBML` round-trip,
+- translated to SQL **only** by the writer for the matching dialect
+  (`@postgres:` clauses appear in PostgreSQL output, never in SQLite output, and
+  vice-versa).
+
+## Grammar
+
+A directive is a single line, matched on its trimmed content:
+
+```
+@<namespace>[(<target>)]: <args>
+```
+
+| Part | Rules |
+|------|-------|
+| `namespace` | `^[a-z][a-z0-9_]*$` — e.g. `postgres`, `sqlite`. Future dialects allowed. |
+| `(target)` | Optional. A **column name** only, valid only on a directive line inside a table body. Bare or single/double quoted. |
+| `args` | Everything after the first `:`, trimmed. Otherwise preserved **verbatim**. Must be non-empty. |
+
+The **key** of a directive is derived: the lowercased first whitespace-delimited
+token of `args` (`partition by RANGE (created_at)` → `partition`). It drives
+duplicate detection and writer dispatch.
+
+## Location
+
+Where the line appears determines which object it attaches to:
+
+| Position in the file | Attaches to |
+|----------------------|-------------|
+| Before the first `Table {` | database (`db.Metadata`) |
+| Table body, no `(target)` | that table |
+| Table body, `(col)` target | column `col` of that table (error if `col` is unknown) |
+| Inside an `indexes { }` block | the **most recently listed** index entry in that block; `(target)` is not allowed |
+
+```dbml
+@postgres: search_path myapp          -- database
+
+Table myapp.events {
+  id bigint [pk]
+  created_at timestamp [not null]
+  @postgres(id): identity always      -- column "id"
+  @postgres: partition by RANGE (created_at)   -- table
+  @postgres: tablespace fast_data              -- table
+  @sqlite: without rowid                        -- table
+
+  indexes {
+    (created_at) [name: 'idx_events_created']
+    @postgres: with (fillfactor=90)   -- index "idx_events_created"
+    @postgres: tablespace idx_space    -- index "idx_events_created"
+  }
+}
+```
+
+## Duplicate policy
+
+- **Repeatable by default** — every directive with the same `(namespace, key)` at
+  one location is kept, in source order.
+- **Singletons** raise a line-numbered error on a second occurrence at the same
+  location. Current singletons: `postgres` `partition`, `tablespace`, `inherits`,
+  `storage`, `compression`, `identity`; `sqlite` `without`, `strict`, `collate`.
+
+## Strict mode
+
+CLI flag `--strict-directives` (also `ReaderOptions.StrictDirectives` /
+`WriterOptions.StrictDirectives`):
+
+- **Reader**: an unknown namespace or key is a hard error. Without strict mode it
+  is stored and preserved silently, and round-trips unchanged.
+- **PostgreSQL / SQLite writer**: a directive for **that** writer's own dialect
+  whose key it cannot translate is a hard error. Without strict mode, translatable
+  keys are emitted and the rest are skipped. Directives for other dialects are
+  always ignored, never emitted.
+
+## Errors
+
+All are line-numbered (`dbml: line N: …`):
+
+- no colon, or empty `args`
+- namespace empty or not matching `[a-z][a-z0-9_]*`
+- `(target)` naming an unknown column, or used at the top level / in an `indexes` block
+- a directive in the catalog used at a location it is not valid for
+- duplicate singleton at the same location
+- (strict mode) unknown `(namespace, key)`
+
+## Supported directive matrix
+
+### `@postgres`
+
+| Key | Locations | SQL emitted | Notes |
+|-----|-----------|-------------|-------|
+| `partition` | table | `PARTITION BY <args>` appended to `CREATE TABLE` | e.g. `@postgres: partition by RANGE (created_at)` |
+| `inherits` | table | `INHERITS (<args>)` — args verbatim | |
+| `with` | table, index | `WITH (<params>)` | On an index, wins over `WITH` derived from the index comment. `@postgres: with (fillfactor=90)` |
+| `tablespace` | table, index | `TABLESPACE <name>` | Emitted after `WITH`, before `WHERE` on indexes |
+| `storage` | column | `STORAGE <mode>` in the column definition | e.g. `@postgres(blob): storage external` |
+| `compression` | column | `COMPRESSION <method>` | |
+| `identity` | column | `identity always` → `GENERATED ALWAYS AS IDENTITY`; `identity default` / `identity by default` → `GENERATED BY DEFAULT AS IDENTITY` | |
+
+### `@sqlite`
+
+| Key | Locations | SQL emitted | Notes |
+|-----|-----------|-------------|-------|
+| `without` | table | `WITHOUT ROWID` table option | `@sqlite: without rowid` |
+| `strict` | table | `STRICT` table option | `WITHOUT ROWID` is emitted before `STRICT` |
+| `collate` | column | ` COLLATE <name>` in the column definition | e.g. `@sqlite(name): collate NOCASE` |
+
+Unknown namespaces and keys not in these tables are still preserved losslessly
+(and round-trip through the DBML writer) whenever strict mode is off.