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. Choosing tags for an inference request and exposing them through control APIs are handled by separate integration layers.