feat(pdo): add set-fetch-mode statement wrapper

[Chemaclass]

Wraps PDOStatement::setFetchMode as pdo/set-fetch-mode: (set-fetch-mode stmt mode & args). Dispatches on the number of extra args to cover all PDO arities (mode alone; + colno for FETCH_COLUMN; + class + ctor-args for FETCH_CLASS/FETCH_INTO). Extra args go through phel->php, and the statement is returned so it threads through ->.

• Removed from the "Not implemented yet" block in src/pdo/statement.phel.
• Tests: set-fetch-mode-column-returns-scalar (FETCH_COLUMN) and set-fetch-mode-num-returns-positional-row (FETCH_NUM). They read through the raw fetch because pdo/fetch always requests FETCH_ASSOC.
• README API table + CHANGELOG ## [Unreleased] updated.

Polish pass: no refactor needed - case on arg count keeps the variadic dispatch flat.

composer test green (43 assertions).

Closes #13