ADR-0032 §10.3 says cte_bindings lives on the scope frame, with
inner subqueries free to declare their own CTEs that shadow outer
ones. The grammar didn't actually admit nested WITH inside
SQL_SELECT_COMPOUND — a real ADR-vs-implementation gap.
Closes the gap by making SQL_SELECT_COMPOUND a Choice between a
WITH-prefixed form and a plain form. The naive Optional-prefix
approach silently broke the paren-vs-subquery dispatch in
sql_expr.rs's PAREN_GROUP: Optional matches 0 bytes, committing
the Seq, so SELECT_CORE's NoMatch on `(a + b)` became Failed and
the Choice couldn't fall through to or_expr. The Choice-fronted
form keeps the fast NoMatch on non-WITH non-SELECT first tokens.
Side effect: scalar subquery / IN / EXISTS / derived-table
bodies now admit a leading WITH too, which matches standard SQL.
Updated two tests that were guarding the old `(WITH …)` rejection
behavior. Added one new harvest test exercising nested-WITH inside
a CTE body — the harvest's `expand_binding` mechanism already
handled the data correctly; the grammar gap was the sole blocker.
Test totals: 1414 → 1415 passing (+1 nested-with-in-cte test).
Clippy clean.
claude@clouddev1
fd259048da