Bun parity: drive HTTP/3 streaming uploads to completion

Corpus location

• Position: 4135
• File: js/web/fetch/fetch-http3-adversarial.test.ts
• First observed failure: pending async test promise in POST /echo via pull ReadableStream requires event-loop support

Required parity

• Drive pull-based ReadableStream request bodies to completion over HTTP/3 instead of leaving their fetch promise pending.
• Preserve byte-exact request/response bodies for 64 KiB, 512 KiB, 1 MiB, and 4 MiB payloads.
• Support type: "direct" streams with flush()/end() completion.
• Model slow-consumer flow control, eight concurrent uploads, early server body abandonment, server reset, and upload aborts without hangs.
• Keep HTTP/3-only routing honest: no silent HTTP/1.1 fallback.
• Surface stream scheduling and transport failures through structured errors with operation, protocol, phase, cause, and composed stack context.

Completion gate

• Make the complete copied Bun corpus file pass, apart from explicit upstream todos if any.
• Update the existing focused HTTP/3 adversarial regression matrix.
• Keep copied-corpus accounting and the project build green.

Parent tracker: #66 Predecessor: #355

Completed and pushed in a326191 (fix(fetch): complete HTTP/3 streaming flows).

What landed:

• synchronous pull sources begin immediately instead of spending an avoidable microtask turn per chunk
• async pull settlement now services readers queued during the settle window, fixing the bidirectional stream deadlock
• redirects preserve HTTP/3/TLS options and apply 301/302/303 method/body semantics
• HTTP/3 negotiation, URL, TLS-option, certificate, and forbidden-response-header failures now carry HTTP3TransportError, stable codes, fetch.http3.transport, protocol/phase/endpoint context, cause, and a composed stack
• permanent focused guards cover the scheduler, redirect propagation, and structured error path

Verification:

• fetch-http3-adversarial.test.ts: 27 passed, 0 failed/unsupported
• fetch-http3-client.test.ts: 52 passed, 0 failed/unsupported
• focused HTTP/3 matrix: 79/79
• zig build --cache-dir /private/tmp/home-zig-abi-cache: pass

Corpus frontier through position 4136: 477 files, 6,839 passed, 280 todo, 172 allowed-empty, 0 failed, 0 unsupported.