[rhnvrm]

CI fails before running tests because minio/minio:latest is unavailable. Replace the local/CI fixture with the official chrislusf/seaweedfs:4.48 image running in single-process mini mode.

• Seeds public local-test credentials and testbucket, waits for authenticated S3 bucket access with bounded retries, and binds the S3 port to loopback.
• Explicitly disables auto-creating buckets on upload and recursively deleting non-empty buckets to exercise ordinary S3 behavior.
• Shares environment settings between Nix and setup/test commands, runs the suite with the race detector and disables Go's test cache (-count=1), and collects diagnostics/cleans up in CI.
• Restores the original ACL, tagging, versioning and SSE-S3 integration tests. No backend capability skips are added; the pre-existing POST-tagging integration skip remains.
• Adds always-running local HTTP protocol tests for request headers, signed POST policy conditions, XML serialization, response parsing and version query parameters.
• Updates development documentation and commands to use SeaweedFS. No production library code or SSE-C implementation changes.

Validation: local protocol tests pass under -race; go vet ./..., shell syntax checks, Compose YAML parsing and git diff --check pass. The full suite passed against SeaweedFS in GitHub Actions: https://github.com/rhnvrm/simples3/actions/runs/39753774801.

Merge this infrastructure change first, before SSE-C support in #52.