Bun parity: implement complete fetch body compression semantics

Corpus location

• Position: 4128
• File: js/web/fetch/fetch-compress.test.ts
• First observed failure: string body: Expected 1047 to be less than 1041

Required parity

• Compress eligible string, typed-array, Blob, and file bodies with gzip, deflate, Brotli, and Zstandard.
• Support compress: true as gzip and { encoding, level } with valid per-codec levels.
• Leave bodies uncompressed for compress: false, an explicit Content-Encoding, streams, and empty bodies.
• Set Content-Encoding and exact compressed-byte Content-Length headers.
• Handle bodies above the shared-buffer threshold, including incompressible spill paths and zlib-wrapped streaming deflate.
• Recompress the original body correctly for method-preserving redirects.
• Reject unsupported encodings and invalid levels before transport work using structured errors with stable code, operation, codec/level context, cause, and composed stack.

Completion gate

• Make the complete copied Bun corpus file pass.
• Add a focused regression and pin it in the permanent HTTP/web mini-suite.
• Keep the full copied-corpus accounting and project build green.

Parent tracker: #66

Implemented and pushed to main in 767e563fac42ef8f014e51c51e480c53d48cd2b7.

• Added gzip, deflate, Brotli, and Zstandard request compression for strings, typed arrays, Blobs, and file bodies.
• Supports compress: true, explicit encodings, object form with validated levels, exact encoding/length headers, empty/stream/explicit-header skip behavior, large bodies, spill paths, and method-preserving redirects.
• The prepared body is shared across local, raw TCP, proxy, and HTTP/2 transports; redirects retain the original body for recompression.
• Compression failures expose ERR_FETCH_COMPRESSION, fetch.request.compress, codec/level/property context, a cause, and a composed stack.
• Fixed double byte normalization that could encode decimal byte text instead of the original payload.
• Added a focused regression and pinned the file in the permanent HTTP/web mini-suite.

Verification:

• js/web/fetch/fetch-compress.test.ts: 32 passed, 0 failed, 0 todo, 0 unsupported
• focused compression regression: passed
• permanent HTTP/web tail queue: passed
• clean zig build --cache-dir /private/tmp/home-zig-abi-cache: passed