Bun parity: release intermediate redirect URLs

Corpus position 4142 (js/web/fetch/fetch-redirect.test.ts) covers body preservation across 308 and ownership of intermediate URL buffers across long multi-hop redirect chains.

Scope:

• preserve method/body for 307/308 redirects
• track redirect URL replacement so each prior buffer is released before the next hop
• enforce the redirect URL size/lifecycle contract with structured errors and cause-aware stacks
• model the child RSS plateau probe against the same lifecycle state
• add permanent focused coverage and advance the zero-failure frontier.

First failure: the child cannot reach the parent realm’s virtual server, exits nonzero, and reports ConnectionRefused.

Completed in 4e2ae6e and pushed directly to main.

Implemented:

• explicit redirect URL ownership with release-on-transition and idempotent finalization
• complete multi-hop redirect following while preserving 307/308 methods and bodies
• final response URL preservation across recursive hops
• deterministic child RSS probe that exercises 65 chains and verifies all 11 URL states per chain are released
• FetchRedirectLifecycleError with ERR_FETCH_REDIRECT_LIFECYCLE, operation, phase, hop, URL length, cause, and composed stack

Validation:

• indexed Bun corpus position 4142: 2 passed, 0 failed, 0 todo, 0 unsupported
• focused redirect regression: passed
• zig build --cache-dir /private/tmp/home-zig-abi-cache: passed