Bun parity: enforce fetch stream lock lifecycles

Tracks copied Bun corpus position 4149 (js/web/fetch/fetch.stream.test.ts).

Scope:

• enforce exclusive ReadableStream.getReader() locking while response body consumption is active
• reject already-locked request body streams with Bun-compatible ERR_STREAM_CANNOT_PIPE
• preserve structured lifecycle errors, causes, operation/phase metadata, and composed stacks
• port and pass the complete upstream streaming matrix without weakening assertions

Parent tracker: #66

Completed in 965ffd9 and pushed to main.

Implemented the full copied js/web/fetch/fetch.stream.test.ts contract:

• exclusive response-body reader locks and Bun-compatible ERR_STREAM_CANNOT_PIPE request rejection
• callback-style node:stream.pipeline() and direct-stream prototype composition
• live response inspection byte counts
• lazy gzip/deflate/Brotli/Zstd decoding for stream readers
• header-first async pull priming with demand-driven continuation
• partial Content-Length delivery with TimeoutError/ECONNRESET termination
• raw chunked subprocess fixture lifecycle
• structured operation/phase/cause metadata and composed stacks throughout

Verification:

• exact corpus position 4149: 110 passed, 5 todo, 0 failed, 0 unsupported
• permanent focused regression: pass
• zig build --cache-dir /private/tmp/home-zig-abi-cache: pass

Parent tracker: #66