feat(pdo): statement-level error-code/error-info via polymorphic dispatch

[Chemaclass]

Wraps PDOStatement::errorCode / errorInfo. Same approach as #12: instead of new names colliding in the single phel.pdo namespace, error-code / error-info now dispatch on the handle.

(pdo/error-code conn)   ;; connection (unchanged)
(pdo/error-code stmt)   ;; statement

Generalized the attr-target helper from #12 into handle-target (resolves :pdo vs :stmt), now shared by the attribute and error accessors.

• Removed both methods from the "Not implemented yet" block in src/pdo/statement.phel.
• Tests: statement-error-code-on-constraint-violation and statement-error-info-on-constraint-violation, mirroring the existing connection-level error tests (UNIQUE violation via a prepared statement). Existing connection error tests still green.
• README + CHANGELOG (### Changed) updated.

Polish pass: renamed attr-target -> handle-target and reused it, so error and attribute accessors share one resolver.

composer test green (45 assertions).

Closes #14