S3 compatibility and openapi spec.

docs/s3-compatibility.md

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+# S3 Compatibility Matrix
+
+This project is S3-compatible for a focused subset of operations.
+
+## Implemented
+
+### Service and account
+- `GET /` list buckets
+
+### Bucket
+- `PUT /{bucket}` create bucket
+- `HEAD /{bucket}` head bucket
+- `DELETE /{bucket}` delete bucket (must be empty)
+- `GET /{bucket}?list-type=2...` list objects v2
+- `GET /{bucket}?location` get bucket location
+- `POST /{bucket}?delete` delete multiple objects
+
+### Object
+- `PUT /{bucket}/{key}` put object
+- `GET /{bucket}/{key}` get object
+- `HEAD /{bucket}/{key}` head object
+- `DELETE /{bucket}/{key}` delete object
+- `GET /{bucket}/{key}` supports single-range requests
+
+### Multipart upload
+- `POST /{bucket}/{key}?uploads` initiate
+- `PUT /{bucket}/{key}?uploadId=...&partNumber=N` upload part
+- `GET /{bucket}/{key}?uploadId=...` list parts
+- `POST /{bucket}/{key}?uploadId=...` complete
+- `DELETE /{bucket}/{key}?uploadId=...` abort
+
+### Authentication
+- AWS SigV4 header auth
+- AWS SigV4 presigned query auth
+- `aws-chunked` payload decode for streaming uploads
+
+## Partially Implemented / Differences
+- Exact parity with AWS S3 error codes/headers is still evolving.
+- Some S3 edge-case behaviors may differ (especially uncommon query/header combinations).
+- Admin API is custom JSON (`/_admin/v1/*`).
+
+## Not Implemented (Current)
+- Bucket versioning
+- Lifecycle rules
+- Replication
+- Object lock / legal hold / retention
+- SSE-S3 / SSE-KMS / SSE-C
+- ACL APIs and IAM-compatible policy APIs
+- STS / temporary credentials
+- Event notifications
+- Tagging APIs
+- CORS APIs
+- Website hosting APIs