Bun parity: port fetch stream fast-path semantics

Focused checkpoint under #66, following #373.

Upstream corpus frontier: js/web/fetch/stream-fast-path.test.ts (position 4164).

Scope:

• preserve the upstream test unchanged
• implement the logical fetch/Response body stream fast path exercised by Bun
• retain correct byte delivery, chunk ownership, backpressure/lifecycle behavior, and cleanup
• surface actionable operation/phase/cause metadata with composed stacks on stream failures
• add permanent regression coverage and validate the contiguous tail plus full build

Parent: #66 Predecessor: #373

Completed in 44862b1 (feat(fetch): port Blob stream fast paths) and pushed to main.

Validation:

• upstream js/web/fetch/stream-fast-path.test.ts remains byte-for-byte unchanged
• exact position 4164: 18 passed / 0 failed / 0 todo / 0 unsupported / 0 allowed-empty
• all four Blob-backed conversions (arrayBuffer, bytes, text, blob) now return already-fulfilled tracked Promises without wasting a microtask
• Blob stream MIME metadata survives readableStreamToBlob and ReadableStream.prototype.blob()
• live/pull streams retain the asynchronous reader path and body-lock behavior
• synchronously errored streams reject instead of being mistaken for replayable empty streams
• failures expose ERR_READABLE_STREAM_CONVERSION, operation, phase, original cause, and composed stack
• permanent native bootstrap regression passes
• position 4165 utf8-bom.test.ts: 21 passed / 0 failed / 0 unsupported
• full zig build --cache-dir /private/tmp/home-zig-abi-cache passes

The contiguous frontier is now position 4166, js/web/fetch/wasm-streaming.test.ts. Parent: #66. Predecessor: #373.