V-L2-A1: replace hand-rolled SQL DDL parser with sqlparser crate

[hyperpolymath]

Context

src/codegen/parser.rs::parse_sql_schema is a hand-rolled string scanner: uppercase + split on ; + comma-with-paren-respect. It misclassifies CHECK constraints, schema-qualified names, generated columns, quoted identifiers with whitespace, and any DDL containing semicolons inside comments.

What to do

Replace with the sqlparser crate (sqlparser = "0.50") using its Postgres + SQLite dialects. Migrate the existing ParsedSchema / TableDef / ColumnDef IR by walking sqlparser::ast::Statement::CreateTable. Drop split_respecting_parens.

Acceptance

• sqlparser dependency added
• Existing tests pass
• New tests: schema-qualified names (s.t), quoted identifiers with spaces, CHECK clauses, GENERATED columns