fix: encode native multipart names and line breaks correctly

[OskarEichler]

Fixes

• Escape quotes/CR/LF in native multipart field names and filenames.
• Normalize field-name and string-value line endings to CRLF without modifying file bytes.
• Reject file MIME types containing CR/LF before uploading malformed multipart headers.
• Preserve field order, duplicate fields, Unicode, literal percent/backslash values, empty filenames and file metadata defaults.

Compatibility / breaking changes

Names and text values containing line breaks now use browser-compatible multipart encoding; quotes in parameters become percent-encoded. Invalid multiline MIME types now reject. File bytes, public signatures, buffer strategy and dependency versions are unchanged. This does not implement streaming uploads or bound upload memory.

Verification

24 external native serialization checks pass, executing the actual Swift/Kotlin serializers and comparing bytes with Node FormData (except retaining the existing explicit empty filename, which Node omits). 12 assertions fail on the original source. Both native examples build with tracing enabled; no physical device/network upload claim. No test/spec files changed.

Algorithm reference: HTML multipart encoding.

Fixes #224.

Consumer follow-up verification

The combined fixes are backported to an immutable 1.6.1 artifact. Both consuming app Android Debug variants, both iOS Debug Simulator variants, four production Metro bundles, 13 web targets, four browser-extension builds, lint and immutable installation pass. All 274 installed non-metadata files match the artifact. This is build verification, not a physical-device authentication/upload certification.

[vercel]

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