feat(analysis): allow grouped counts and cast-typed projections

[omer-cengel]

Summary

Queries can now report grouped counts and computed values. An aliased COUNT(*) may appear beside other SELECT items, so SELECT city, COUNT(*) AS venues ... GROUP BY city compiles, and a projection whose type is stated by a cast, such as SUM(amount)::numeric AS total, becomes a typed result component.

Changes

• An aliased COUNT(*) is no longer required to be the only SELECT item; it is a non-null Long component in its own position beside direct columns, wildcards, cast projections, or other counts.
• A projection written as <expression>::type AS alias or CAST(<expression> AS type) AS alias (with or without AS) becomes a nullable component of the cast type, including declared enum types and one-dimensional arrays.
• New diagnostics: A cast projection requires a result alias, such as SUM(amount)::numeric AS total. and Result column '<alias>' has unsupported cast type <TYPE>.
• Analyzer tests cover count position, grouped counts with predicate parameters, both cast and alias spellings, enum and array casts, and the rejections; PostgreSQL integration tests execute a generated grouped count and a cast SUM, including the null result when no row matches.
• docs/queries.md documents both projections, their diagnostics, and the covering tests.

Scope and non-goals

• The cast operand is not analyzed; it reaches the database as written, and PostgreSQL checks its column references and functions.
• GROUP BY and HAVING remain unanalyzed and are passed through as written.
• Expressions without a cast (for example COUNT(column), lower(name), FILTER/OVER forms) and row-type casts stay rejected as unsupported SELECT expressions.
• A placeholder in a projection, including a cast placeholder such as $1::int AS x, stays rejected.
• RETURNING, code generation, and the runtime are unchanged.