Release v0.2.26

Closes issue #15: narrow whitelist for session unserialize.

v0.2.25 set allowed_classes => false in all session decode paths,
converting stdClass (the most common session object shape — json_decode
output, OAuth tokens, API profiles) to __PHP_Incomplete_Class and
breaking real apps. v0.2.26 narrows the whitelist to ['stdClass']
specifically — zero magic methods means no gadget chain — while keeping
every other class refused.

PHPStan level 10 clean. 385 unit + 147 integration tests pass.