CacheLayer 3.3: expand atomic cache backend coverage

[abmmhasan]

Summary

Expand the optional Cache::atomic() capability to additional backends while preserving the existing full three-operation contract (setIfAbsent, strict compareAndSet, getAndDelete).

Batch 1 — WeakMap

• Add process-local atomic capability.
• Preserve strict PHP comparison semantics and collected-reference behavior.

Batch 2 — Memcached

• Add native CAS-backed conditional replacement.
• Add add()-based atomic claim.
• Add CAS tombstone consume semantics so getAndDelete() cannot delete a concurrent replacement.
• Preserve namespace and tag-generation validity.

Batch 3 — File / PHP files

• Add deterministic per-key flock() coordination.
• Make ordinary save() / deleteItem() mutations share the same key lock as atomic operations.
• Keep reads lock-free over atomic rename replacement.

Batch 4 — PDO

• Add driver-qualified atomic capability for SQLite, PostgreSQL, MySQL, and MariaDB.
• SQLite uses BEGIN IMMEDIATE.
• PostgreSQL/MySQL/MariaDB use transactions with row locking and conflict-safe inserts.
• Other PDO drivers continue to return no atomic capability.
• Reject atomic operations inside caller-owned active PDO transactions.

Tests / docs

• Add backend-expansion contract tests.
• Update SQLite capability expectations.
• Document the expanded backend matrix and consistency domains.

APCu, ScyllaDB, Null, and Tiered remain intentionally unsupported.

[github-advanced-security]

PHPStan found more than 20 potential problems in the proposed changes. Check the Files changed tab for more details.