Bun parity: complete client fetch API semantics

Upstream coverage

• js/web/fetch/client-fetch.test.ts (corpus position 4120)

Current gap

The first failing assertion reports function signature: Expected 2 to be 1, indicating that Home's fetch-compatible callable metadata does not yet match Bun's public API contract.

Required parity

• match Bun's fetch.length and callable surface metadata
• preserve existing request/response behavior while correcting the public signature
• run the complete upstream file and resolve each remaining semantic mismatch logically
• model any failures with stable structured Home error identities, causal context, and useful stacks
• add a focused regression and pin the upstream file in the permanent web queue

Blocked by/completes the next ordered step after #347.

Implemented and pushed to main in c856f35 (fix(fetch): complete client request semantics).

What landed:

• Bun-compatible fetch.name / fetch.length and invalid-input validation
• structured ERR_FETCH_INPUT, ERR_FETCH_CONNECT, and ERR_FETCH_REDIRECT failures with operation context, causes, and composed stacks
• Node HTTP server sync/async disposal for using / await using
• byte-preserving request/response bridging, request data/end, req.pipe(res), and redirect body replay
• HTTP response MIME normalization without altering byte-exact multipart boundary values
• UTF-8 multipart field-name and filename decoding
• structured ERR_FORM_DATA_PARSE closing-boundary failures
• distinct locked vs disturbed body semantics through ERR_BODY_STREAM_STATE
• Bun-compatible .resolves.pass() behavior for already-resolved explicit-pass assertions
• focused and permanent-queue regressions

Verification:

• exact corpus position 4120: 29 passed, 2 todo, 0 failed, 0 unsupported
• dedicated client-fetch regression: passed
• permanent HTTP/web tail queue: passed
• clean-cache zig build --cache-dir /private/tmp/home-zig-abi-cache: passed

The ordered frontier now reaches through position 4120. Position 4121 is tracked separately.