Implement iterable BlobPart conversion, MIME and endings normalization, zero-copy Blob composition and slicing, and lazy 64 KiB byte streams. Delegate File streaming and slicing to its backing Blob while preserving browser class identity.\n\nUse immutable shared segments so nested Blob and slice construction do not duplicate payload bytes. Add focused WPT, WebKit, and Firefox regression coverage for constructor ordering, iterator closure, UTF-8 decoding, BYOB reads, cancellation, lifetime safety, and large nested streams.

Replace the ad hoc buffered fetch response object with standard-shaped Headers and Response classes backed by ReadableStream bodies. Preserve host constructors, normalize and validate header values, enforce single-use bodies, and expose native responses with one transport-to-JavaScript copy.

Reuse normalized header views between mutations, compact header storage in place, retain stream chunks only until consumption, and allocate a contiguous body result at most once when required.

Add focused WPT, WebKit, Firefox, and Chromium regression coverage plus JavaScriptCore/QuickJS and initialization tests.

Close byte streams for zero-length BufferSource bodies without enqueueing an invalid empty chunk. Preserve the non-null body and bodyUsed semantics, and add focused regression coverage alongside the WPT empty-response cases.

Resolve percent-encoded and base64 data URLs without entering the network transport. Use a validating WHATWG-style forgiving-base64 decoder, strip URL fragments from payloads, and preserve response metadata through the standard Response path.

Decode into pre-sized native storage and copy the completed body once into JavaScript-owned memory. Cover representative WPT data URL and forgiving-base64 vectors, plus Chromium whitespace cases.

Replace a partial or null host Streams surface as a complete constructor suite so stream products retain compatible instanceof relationships. Preserve a complete suite on repeated initialization.\n\nHarden the native initialization test so N-API failures complete the test promise instead of hanging, and cover partial host replacement plus null handling and cross-constructor compatibility.

Use ReadableByteStreamController.byobRequest views directly and respond with the produced byte count instead of allocating and enqueueing a 64 KiB intermediate chunk. Keep the existing bounded allocation path for default readers.\n\nAdd WPT-derived coverage for small offset BYOB views that cross immutable Blob segment boundaries.

Observe standard ReadableStream consumption paths with weak per-stream state instead of depending on web-streams-polyfill's private _disturbed field. Instrument stream readers and piping once during Fetch initialization, preserve stream identity, and cache the initialized Fetch implementation so repeated initialization does not stack wrappers or replace its internal state.

Treat Headers and Response as an implementation pair when either host global is missing or null. Harden native initialization tests against unhandled N-API failures and add WPT-derived coverage for reads, cancellation, piping, and streams disturbed before Response construction.