Cache identity tags

ObjectKeyIdentity.tags adds opaque string dimensions to cache identity. Tags are copied into an immutable, name-sorted mapping, so insertion order and later caller mutation do not affect equality, hashing, or storage names.

identity = ObjectKeyIdentity(
    chunk_hash=chunk_hash,
    kv_rank=kv_rank,
    model_name=model_name,
    tags={"adapter_revision": "abc123"},
)

L2 adapters use lmcache.v1.distributed.key_codec for canonical persistent names. Untagged keys keep their existing names; tagged keys append a reversible versioned token. The codec does not impose backend-specific name-length limits; storage backends report unsupported names through their normal I/O results.

ObjectKey keeps its flat constructor and accepts tags= for callers that do not construct ObjectKeyIdentity directly. EncodedObjectKey and the internal identity protobuf preserve tags across their existing projections. Multiprocess clients choose tags with string-valued lmcache.tag.<name> entries in IPCCacheServerKey.request_configs. The server applies those tags to every expanded object key and keeps pending transfers isolated by the same mapping. Invalid matching tags reject only their RPC instead of being dropped or leaving the client waiting for a timeout. Control APIs are handled by a separate integration layer.