feat(pdo): add column-meta statement wrapper

[Chemaclass]

Wraps PDOStatement::getColumnMeta as pdo/column-meta: (column-meta stmt column). Returns the column's metadata as a map (keys keywordised via the existing row->map), or nil when no metadata is available (getColumnMeta returned false).

(pdo/column-meta stmt 1)
;; => {:name "name" :table "t1" :pdo_type 2 :len -1 :precision 0 ...}

• Removed from the "Not implemented yet" block in src/pdo/statement.phel.
• Test column-meta-returns-map asserts the driver-stable :name and :table keys (other keys vary by driver/version, so they are not asserted).
• README API table + CHANGELOG ## [Unreleased] updated.

Polish pass: no refactor needed - reuses row->map, consistent with the other readers.

composer test green (47 assertions).

Closes #15