Port Bun S3 storage-class signing semantics

Tracks the focused compatibility tranche for js/bun/s3/s3-storage-class.test.ts under #66 and #81.

Scope:

• validate Bun storage-class enum values at client, file, writer, and request boundaries
• sign x-amz-storage-class across static, instance, and multipart upload paths
• use canonical lowercase storage-class presign query parameters
• expose the default Bun s3 client
• activate the exact copied 9-test fixture and add a permanent Home regression gate

Close once the exact fixture and full Home gate pass.

Completed in 42028172c.

Implemented Bun-compatible storage-class enum validation, signed x-amz-storage-class propagation for static/instance writes and multipart initiation, canonical lowercase presign query parameters, file/writer option precedence, and the default s3 client.

Verification:

• exact copied js/bun/s3/s3-storage-class.test.ts: 9 passed, 0 failed, 0 todo, 0 unsupported
• ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: passed

The permanent regression executes the exact copied fixture. This advances #66 and #81.