feat(cache): add Memcached lock store

[memleakd]

Description

This PR follows up on the new Atomic Locks feature added in #10145 by adding Memcached as a supported lock store.

The initial locks PR intentionally shipped with File, Redis, and Predis support first because Memcached needed a more careful owner-aware design. This PR adds that missing store using the memcached PHP extension and CAS-based release/refresh behavior.

Implementation

This adds:

• CodeIgniter\Cache\LockStores\MemcachedLockStore
• MemcachedHandler::lockStore()
• live Memcached lock tests
• docs and changelog updates

The implementation keeps the structure introduced in #10145:

• Lock behavior lives in a dedicated *LockStore class.
• MemcachedHandler only acts as a LockStoreProviderInterface.
• CodeIgniter\Lock continues to depend on cache lock-store contracts, not concrete cache handlers.

Memcached Behavior

Lock acquisition uses Memcached's atomic add() operation.

Owner-aware release() and refresh() use CAS tokens from Memcached::GET_EXTENDED. This avoids the unsafe get owner -> delete/touch pattern where a lock could expire and be reacquired by another owner between operations.

Memcached does not provide an atomic compare-and-delete command, so release first uses CAS to shorten the TTL for the current owner before deleting the key. This keeps the owner check atomic while staying within Memcached's available operations.

The older memcache PHP extension is not supported for locks because it does not provide the CAS operations needed for owner-aware release and refresh. If MemcachedHandler is running with memcache, resolving locks fails with the normal lock unsupported-store exception.

Research Notes

This follows the same general direction as other ecosystems. Symfony provides a dedicated MemcachedStore in its Lock component [1], [2], and Laravel has a MemcachedLock implementation in its cache lock system.

For the Memcached-specific design, the important primitives are atomic add() for acquisition and CAS tokens for owner-aware updates. PHP exposes CAS metadata through Memcached::GET_EXTENDED, while Memcached’s protocol documents add / CAS as the available race-safe storage primitives.

Testing

Added live Memcached coverage for:

• acquiring a lock
• rejecting competing owners
• owner-aware release
• owner-aware refresh
• active and missing-key force release
• expiry and reacquisition by a new owner
• handler reconnect refreshing the memoized lock store
• normalizing provider-time unsupported-store failures

Checklist

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide