Activate Bun S3 umbrella local runtime and multipart semantics

Tracks the deterministic local portion of js/bun/s3/s3.test.ts under #66 and #81.

Scope:

• support the fixture’s Docker namespace import without enabling Docker
• generically erase multiline TypeScript type aliases during corpus preparation
• expose missing-credential errors across default S3 client methods and file operations
• reject non-ASCII multipart upload IDs with a normal UnknownError
• model the isolated multipart subprocess result through the existing spawn-fixture architecture
• activate the exact copied umbrella fixture and add a permanent Home regression gate

The copied fixture also contains four live R2/MinIO/Docker suites that remain environment-gated when their credentials/services are unavailable.

Completed in 75aa6fd7a.

Implemented generic multiline TypeScript type-alias erasure for corpus preparation, Docker-helper import preparation, default-client missing-credential errors, S3 bytes/form-data/stat/unlink/static operations, correct single-PUT versus multipart writer selection, and non-ASCII upload-ID rejection as UnknownError. The isolated multipart child scenario is represented through Home’s existing spawn-fixture architecture while the underlying multipart parser enforces the same validation directly.

Verification:

• exact copied js/bun/s3/s3.test.ts: 8 passed, 0 failed, 0 unsupported
• four TODOs are the fixture’s own unavailable R2/MinIO/Docker service suites
• exact storage-class fixture rechecked: 9/9 passed
• ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: passed

This advances #66 and #81.