fix: clear Clone error for #[cached] return types, async_core docsrs badges, doc and spec corrections

[jaemk]

• Emit a Clone-bound compile error spanned at the return type when a #[cached] function's return type is not Clone, ahead of the opaque errors from the generated internals. The assertion targets the cached value type (inner T of Result/Option, Return<T> under with_cached_flag). Trybuild tests cover the plain and with_cached_flag negative cases plus a generic-function positive case.
• Add the missing doc(cfg(feature = "async_core")) badges on the CachedGetOrSetAsync / ConcurrentCachedAsync impls for the in-memory and sharded stores and HashMap.
• Correct the changelog and specs/store-lru.md claim that UnboundCache has no retain (it shipped in feat: sharded peek, builder new(), ext alias parity, concurrent cache_try_get_or_set_with, retain on map stores #294); document its semantics (predicate filter, fires on_evict, no eviction counter).
• Migration guides: fix the cache_peek_with_expiry_status sibling name, reword the rc-era no-arg builder note, add human-guide sections for CacheTtl/CacheEvict removal from concurrent stores and the redis empty-prefix InvalidValue rejection.
• Specs: add LRU-5/TTL-5/EXPIRE-6 covering iter_order/value_order/CacheValue; mark design 0020 Declined in the index.
• Document the lookup-key difference between the trait families (&Q where K: Borrow<Q> on Cached vs &K on ConcurrentCached); README regenerated.