feat: ADR-0035 4a — SQL CREATE TABLE grammar shape

The post-CREATE shape (src/dsl/grammar/sql_create_table.rs):
TABLE [IF NOT EXISTS] <name> ( <col-def | table-PK> , … ) [;]
- col-def: <name> <type> [NOT NULL] [UNIQUE] [PRIMARY KEY]
- type: ten keywords + standard-SQL aliases (via from_sql_name) +
  the two-word `double precision` branch + discarded (len[,len]) arg
- table-level PRIMARY KEY (cols) — single and compound
- __rdbms_* target rejected at walk time

DEFAULT/CHECK/table-level UNIQUE shapes are deliberately absent (the
4a.2 constraint slice); FK is absent (4b). 13 accept/reject tests
mirror sql_insert's walk_node harness. Shape only — the CommandNode +
builder + worker wiring follow.

src/dsl/grammar/mod.rs

@@ -28,6 +28,7 @@ pub mod ddl;
 pub mod expr;
 pub mod shared;
 pub mod sql_expr;
+pub mod sql_create_table;
 pub mod sql_delete;
 pub mod sql_insert;
 pub mod sql_select;