In-progress HawDB-owned PostgreSQL SQL/PGQ syntax frontend.
This crate owns tokens, byte spans, structured syntax errors, and syntax ASTs.
Semantic binding and logical-plan lowering belong to hawdb-sql.
This frontend is not yet selected by Database::query_sql*; the existing
relational execution path still uses upstream sqlparser. Syntax acceptance
alone does not qualify a statement family for production execution.
The SQL/PGQ specification defines the compatibility contract. The implementation and routing checklist tracks the remaining parser/binder differential coverage, catalog durability, and shared query-pipeline qualification required before enabling production routing. Parser selection must be explicit by statement family, never a retry after another parser fails.