S3
==

An L2 adapter that stores KV cache objects as S3 objects using the AWS
Common Runtime (CRT).  Works with AWS S3, S3 Express One Zone, and any
S3-compatible endpoint (MinIO, Ceph RGW, etc.).

**Required fields:**

- ``s3_endpoint``: Bucket URL -- either ``"s3://<bucket>"`` or the bare host form
  (used for non-AWS endpoints).
- ``s3_region``: AWS region string (e.g. ``"us-west-2"``).

**Optional fields:**

- ``s3_num_io_threads`` (int, default ``64``): Number of CRT I/O threads.
- ``s3_prefer_http2`` (bool, default ``true``): Negotiate HTTP/2 via ALPN.
- ``s3_enable_s3express`` (bool, default ``false``): Enable S3 Express signing
  for S3 Express One Zone buckets.
- ``disable_tls`` (bool, default ``false``): Bypass TLS when pointing at a
  plain-HTTP endpoint (e.g. a local MinIO).
- ``aws_access_key_id`` / ``aws_secret_access_key`` (string): Static
  credentials; omit both to use the AWS default credential provider chain
  (environment, EC2 instance profile, etc.).
- ``max_capacity_gb`` (float, default ``0.0``): Aggregate capacity used by
  ``get_usage()``.  A value of ``0`` disables aggregate eviction
  (``usage_fraction == -1.0``).

**Configuration examples:**

.. code-block:: bash

    # AWS S3 with default credentials
    --l2-adapter '{"type": "s3", "s3_endpoint": "s3://my-bucket", "s3_region": "us-west-2"}'

    # Static credentials, HTTP/2 disabled
    --l2-adapter '{"type": "s3", "s3_endpoint": "s3://my-bucket", "s3_region": "us-west-2", "s3_prefer_http2": false, "aws_access_key_id": "AKIA...", "aws_secret_access_key": "..."}'

    # Local MinIO over plain HTTP
    --l2-adapter '{"type": "s3", "s3_endpoint": "minio.local:9000", "s3_region": "us-east-1", "disable_tls": true, "aws_access_key_id": "minio", "aws_secret_access_key": "minio123"}'