Run the CAS fencing contract in CI against SeaweedFS

Thanks for the exciting project!

fencing.md lists the three properties celld needs from a store, and warns that a store can accept the conditional headers without applying them and fail late and silently. The test that checks this contract, `put_cas_contract_against_real_bucket`, is gated on `CELLD_CAS_LIVE=1` and needs a real bucket, so it never runs in CI — the fencing contract is only exercised at release time against R2, and the local backend the ltx conformance suite runs against does not implement the conditional writes.

I maintain SeaweedFS. Its S3 gateway applies both preconditions atomically — the `If-None-Match: *` / `If-Match` check runs under a per-object write lock, so it is a real compare-and-swap, not the accept-and-ignore failure mode the doc warns about. That makes the fencing contract testable in CI from a single static binary, no Docker and no cloud credentials:

```sh
curl -sL https://github.com/seaweedfs/seaweedfs/releases/download/4.41/linux_amd64.tar.gz | tar xz
./weed mini -dir=/tmp/celld-s3 -bucket=litestream -s3.port=8333 &
```

The S3 endpoint is ready in under a second. With release 4.41, both suites pass unchanged:

```sh
RUSTYRIVER_MINIO_ENDPOINT=http://127.0.0.1:8333 AWS_ACCESS_KEY_ID=any AWS_SECRET_ACCESS_KEY=any \
cargo test -p celld-ltx --features s3 --test integration_minio
# test result: ok. 2 passed (conformance suite + multipart round trip)

CELLD_CAS_LIVE=1 CELLD_CAS_BUCKET=litestream CELLD_CAS_ENDPOINT=http://127.0.0.1:8333 \
AWS_REGION=us-east-1 AWS_ACCESS_KEY_ID=any AWS_SECRET_ACCESS_KEY=any \
cargo test -p celld put_cas_contract -- --nocapture
# CAS verified on litestream: create / reject-create / update / reject-stale
```

The raw dialect checks out too: PUT with `If-None-Match: *` returns 200 on an absent key and 412 on an existing one; PUT with `If-Match` returns 200 with the current etag and 412 with a stale one, and the rejected bodies never land. `Bucket::open` already selects path-style with a custom endpoint, so nothing in celld needs to change — this is CI wiring only.

Happy to send a PR with the workflow. Either way, fencing.md's support matrix could mention SeaweedFS as a store that implements the conditional writes.

Start with the CI workflow and the existing `put_cas_contract_against_real_bucket` and `integration_minio` tests. Run the supplied SeaweedFS startup and cargo test commands locally, then wire the workflow to download and launch SeaweedFS before running both suites. Done means the CAS contract and S3 integration tests pass in CI without Docker or cloud credentials.

Written by the indexing model from the issue text.