Bun parity: finish FormData multipart serialization matrix

Focused checkpoint under #66, following #375.

Upstream corpus frontier: js/web/html/FormData-multipart-serialization.test.ts (position 4168).

Scope:

• preserve the upstream test unchanged
• port multipart boundary, disposition, filename, MIME, byte, and escaping serialization semantics
• keep FormData/Blob/File ownership and request body behavior consistent
• retain actionable serialization operation/phase/cause metadata with composed stacks
• add permanent regression coverage and validate the contiguous tail plus full build

Parent: #66 Predecessor: #375

Completed in 9dd7d9e (feat(formdata): port byte-exact multipart serialization) and pushed to main.

Validation:

• upstream js/web/html/FormData-multipart-serialization.test.ts remains byte-for-byte unchanged
• exact position 4168 on macOS: 3 passed / 0 failed / 1 platform todo / 0 unsupported / 0 allowed-empty
• multipart serialization is byte-first, preserving arbitrary binary values without UTF-8 replacement
• boundary shape, CRLF layout, quote/CR/LF escaping, anonymous filename="", filename overrides, and terminal boundary match Bun
• binary parts receive application/octet-stream; bare textual MIME types gain charset=utf-8
• anonymous Blob parsing restores Bun’s File-with-undefined-name and empty-type semantics
• Response and Request carry byte bodies; Response.formData() parses bytes directly
• missing Bun.file parts throw ENOENT wrapped as ERR_FORM_DATA_SERIALIZE with operation/phase/cause/composed stack
• permanent all-256-byte and error-contract regressions pass
• adjacent unchanged FormData.test.ts: 129 passed / 0 failed
• positions 4170 and 4171 pass 12/12
• full zig build --cache-dir /private/tmp/home-zig-abi-cache passes

The contiguous frontier is now position 4172, js/web/intl/intl.test.ts. Parent: #66. Predecessor: #375.