V4

[unkn0wn-root]

This PR introduces CASCache v4 and replaces the v3 implementation with a smaller API built around typed caches, snapshots, explicit backends, and invalidation fences.

The main read path is now Cache.Load. Concurrent misses for the same key share one loader call. Caller cancellation, loader timeouts, panic recovery, and invalidation during an active load are handled without returning a value that can no longer be confirmed as current.

Main changes

• Move the module to github.com/unkn0wn-root/cascache/v4.
• Add local, Redis, and shared Redis backends behind one backend interface.
• Add snapshot-based conditional writes with clear stored, conflict, rejected, and disabled outcomes.
• Add a standalone invalidator for services that do not need the cached value type.
• Add typed keys, TTL jitter, metrics callbacks, and observer hooks.
• Reject known Redis replica-read configurations because replication lag can return stale values with matching fences.
• Add backend conformance tests and provider tests.
• Add local and Redis benchmarks for reads, sets, and invalidation.
• Rewrite the README and examples around the v4 API.

Breaking changes

• Imports must use the /v4 module path.
• The old batch, read-guard, version-store, and Redis APIs have been removed.
• Storage keys now use the cas:v4 prefix and entries use wire format version 4. Existing v3 cache entries are not reused.
• Backends, stores, and Redis clients remain owned and closed by the caller.