feat(pdo): statement-level get/set-attribute via polymorphic dispatch

[Chemaclass]

Wraps PDOStatement::getAttribute / setAttribute. Rather than introduce new names that collide with the connection-level accessors in the single phel.pdo namespace, get-attribute / set-attribute now dispatch on the handle (per the option suggested in #12):

(pdo/get-attribute conn \PDO/ATTR_ERRMODE)   ;; connection (unchanged)
(pdo/get-attribute stmt \PDO/ATTR_CURSOR)     ;; statement

A private attr-target helper resolves :pdo vs :stmt from the struct type (statement?), keeping both accessors a one-liner. (declare statement?) added before the (load "pdo/statement") so the predicate resolves cross-file.

Note on the test driver

Statement attributes are driver-specific. SQLite (the test driver) doesn't support them: getAttribute raises IM001 and setAttribute is a no-op returning false. The tests therefore prove routing - that the statement branch reaches the PDOStatement (observable via the \PDOException) while the connection branch keeps returning values. The wrapper is fully functional for drivers that support statement attributes (e.g. MySQL).

• Removed both methods from the "Not implemented yet" block in src/pdo/statement.phel.
• Tests: set-attribute-dispatches-to-statement, get-attribute-dispatches-to-statement (connection + statement branches). Existing attribute-roundtrip still green.
• README + CHANGELOG (### Changed) updated.

Polish pass: extracted attr-target to remove the get/set duplication.

composer test green (41 assertions).

Closes #12