Bun parity: isolate split CONNECT proxy envelopes

Corpus position 4140 (fetch-proxy-connect-tunnel-split-envelope.test.ts) guards Bun #30381.

Scope:

• parse a CONNECT 200 envelope incrementally across TCP reads
• discard proxy-only headers before parsing the tunneled upstream response
• decode the upstream chunked body and recognize its terminator without waiting for FIN
• prevent Proxy-Agent/Connection leakage into the user response
• model the subprocess fixture through the same parser
• surface structured tunnel parse errors with operation, phase, byte counts, cause, and composed stack
• add permanent focused coverage and advance the zero-failure frontier.

First failure: the child fixture produces no JSON response, leading to JSON Parse error: Unexpected EOF.

Completed and pushed in a019131 (fix(fetch): isolate CONNECT tunnel envelopes).

Implemented:

• incremental CONNECT response parsing across arbitrary TCP chunk boundaries
• strict successful CONNECT status validation
• proxy envelope buffer isolation before upstream parsing
• upstream HTTP/chunked response parsing that completes on the terminal chunk without waiting for FIN
• proxy-only header exclusion from the user-facing response
• deterministic subprocess fixture execution through the same parser
• structured FetchProxyTunnelError with stable code, operation, phase, endpoint, received/proxy byte counts, cause, and composed stack
• permanent focused regression

Verification:

• upstream split-envelope regression: 1 passed, 0 failed/unsupported
• focused split CONNECT regression: pass
• full zig build: pass

Corpus frontier through position 4140: 481 files, 6,875 passed, 281 todo, 172 allowed-empty, 0 failed, 0 unsupported.