Bun parity: implement fetch.preconnect lifecycle

Corpus position 4139 (js/web/fetch/fetch-preconnect.test.ts) covers preconnected socket reuse and recovery.

Scope:

• lower side-effect import "harness" correctly
• implement fetch.preconnect() URL validation and local TCP connection establishment
• reuse healthy preconnected sockets for fetch
• replace sockets closed via end/terminate/shutdown before, during, or after request startup
• support --fetch-preconnect=<url> subprocess behavior
• provide structured preconnect errors with operation, URL/phase, cause, and composed stack
• add permanent focused coverage and advance the zero-failure frontier.

First blocker: side-effect import lowering currently produces invalid import-call syntax.

Completed and pushed in 4f84bcd (feat(fetch): implement preconnect lifecycle).

Implemented:

• correct side-effect harness import lowering
• synchronous URL validation for fetch.preconnect()
• a reusable local preconnect pool keyed by origin
• request/response hook attachment to healthy preconnected sockets
• one-shot fresh-socket retry when a pooled connection closes without response bytes
• protection against paired end + close notifications consuming the retry twice
• Bun socket terminate() parity and --fetch-preconnect=<url> child behavior
• structured FetchPreconnectError with stable code, operation, phase, endpoint, cause, and composed stack
• permanent full-file focused regression

Verification:

• fetch-preconnect.test.ts: 12 passed, 0 failed/unsupported
• focused preconnect lifecycle: pass
• full zig build: pass

Corpus frontier through position 4139: 480 files, 6,874 passed, 281 todo, 172 allowed-empty, 0 failed, 0 unsupported.