Add encrypted cookies with key rotation

[TomK]

Adds EncryptedCookieHandler, which encrypts cookie values through the CookieJar handler chain:

$jar->addHandler(new EncryptedCookieHandler(Encrypter::fromConfig($config), ['plain-text-cookie']));

Encryption\Encrypter uses XChaCha20-Poly1305 via ext-sodium (or paragonie/sodium_compat, both suggested, not required). security.encryption_key can be a list: the first key encrypts and every key decrypts, so a key can be rotated without signing everyone out. Payloads match cubex/framework 2.7, so apps on either can share cookies with the same keys.

A cookie that fails to decrypt is left out of the jar rather than read as an empty string. To support that, a handler can now throw InvalidCookieException from decodeValue(), and hydrate() drops that cookie.

Test plan

PHPUnit passes on PHP 8.2 and 8.5 (75 tests). New tests cover round trips through a request and response, decrypting with a previous key, dropping forged and retired-key cookies, plain-text exclusions, and the encrypter's tamper, malformed-payload and key-validation cases.

🤖 Generated with Claude Code

[TomK]

Closing: no Cubex 4 app uses encrypted cookies today. If one needs them, the encrypter belongs in its own package (e.g. packaged/encrypt) alongside a cookie handler, with only the InvalidCookieException hook here. The code stays on the encrypted-cookies branch.