feat(expander): Add star expander for SELECT * and RETURNING * (PostgreSQL, MySQL, SQLite)

[kyleconroy]

Summary

Adds a new expander package under internal/x/expander/ that expands * expressions in SQL queries to explicit column names by preparing the query against a database.

Features

• Expands SELECT * to explicit column list
• Preserves table prefix for qualified stars (e.g., authors.* → authors.id, authors.name, authors.bio)
• Handles RETURNING * in INSERT/UPDATE/DELETE statements (PostgreSQL)
• Recursively expands CTEs, including dependent CTEs where later CTEs reference earlier ones
• Supports subqueries in FROM clause
• Database-agnostic design via ColumnGetter interface

Supported Databases

• PostgreSQL - Uses pgxpool connection to get column metadata via Prepare
• MySQL - Uses forked driver (github.com/sqlc-dev/mysql) with StmtMetadata interface
• SQLite - Uses native ncruces/go-sqlite3 API for column metadata

Example

-- Input
SELECT * FROM authors

-- Output
SELECT id, name, bio FROM authors

-- Input (PostgreSQL)
WITH a AS (SELECT * FROM authors), b AS (SELECT * FROM a) SELECT * FROM b

-- Output
WITH a AS (SELECT id, name, bio FROM authors), b AS (SELECT id, name, bio FROM a) SELECT id, name, bio FROM b

Test plan

• PostgreSQL tests (14 test cases) - CTEs, RETURNING *, qualified stars
• MySQL tests (8 test cases) - qualified stars, table-scoped expansion
• SQLite tests (8 test cases) - basic expansion, qualified stars
• COUNT(*) correctly left unexpanded across all databases
• All tests pass with proper SQL formatting (spaces after commas)

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