Add end-to-end filesystem checks and S3 copy compatibility

[aribon-cs]

• Add end-to-end integration coverage for filesystem-backed S3 operations using temporary files
• Verify payload-based MD5 separately from ETag to avoid incorrect checksum assumptions
• Extract reusable app/bootstrap wiring into src/lib.rs so integration tests can boot the real server
• Implement S3-compatible CopyObject handling via x-amz-copy-source
• Add minimal GET ?acl compatibility to support Flysystem/AWS S3 copy flows
• Update integration tests to exercise real copy semantics instead of emulated read/write copy

Why

• Flysystem checksum defaults to ETag, while FerroS3 ETag is not a content MD5
• Flysystem rename/move relies on S3 copy semantics, which FerroS3 did not previously support
• The previous copy test validated a helper path, not the actual S3 adapter behavior

Validation

• • cargo test