feat(poet): add Switch statement, remove Func.Body field

Add Switch statement AST node:
- Switch: switch statement with optional init and expression
- Case: case clause with values (empty for default)

Breaking change to Func struct:
- Remove Body field to force use of Stmts
- Update all usages in generator.go to use Stmts with RawStmt

This enforces consistent use of the structured statement API
instead of allowing raw body strings as an escape hatch.

Author: claude