fix(cache): harden providers and default cache handling

Make cache-write failures non-fatal in GetOrSet/Remember, replace the
unsynchronised default cache with an atomic pointer, fix tag-index leaks
and write-lock-on-read in the memory provider, add a janitor, closed
state and byte copies, hash/namespace memcache keys with CAS tag index,
gate Clear behind AllowFlush, return ErrNotFound without the key, and
allowlist Redis stats. Mark audit status.

pkg/cache/example_usage.go

@@ -1,3 +1,7 @@
+//go:build ignore
+
+// Examples are excluded from the build: they call log.Fatal and are not part of the API.
+
 package cache
 
 import (