Watch fan-out cache

The watch fan-out cache lives in its own Go module at the top of the repo (cache/). For full coverage see modules/cache.

Quick summary

• Buffers a single upstream clientv3.Watch per key prefix.
• Fans out events to many local watchers without consuming additional cluster watch slots.
• Keeps a bounded ring buffer of recent events so newly-attached watchers can catch up without resyncing from the keyspace.
• Uses RequestProgress to keep the watch's high-water mark current — therefore cannot be placed behind the gRPC proxy.

This module is recommended for in-process workloads with thousands of watchers (e.g. an API server colocated with its etcd client). The gRPC proxy remains the right tool for out-of-process consolidation.

For details see modules/cache.