Bun parity: preserve basic permessage-deflate behavior

Parent: #66\nPredecessor: #397\n\nUpstream position 4211: js/web/websocket/websocket-permessage-deflate-simple.test.ts.\n\nContracts:\n- Bun.serve negotiates permessage-deflate and exposes it through WebSocket.extensions\n- client-to-server compressed echo preserves text payloads\n- server-to-client compression preserves a 10,000-byte repetitive payload\n- the exact upstream file remains pinned in the permanent focused JSC regression suite\n\nAcceptance: exact upstream file passes 2/2 with zero failed, todo, unsupported, or allowed-empty cases; positions 4202-4211 pass 61/61.

Completed by 464eea5 (test(websocket): pin basic permessage deflate).\n\nThe exact upstream simple permessage-deflate file is now part of the permanent focused JSC suite.\n\nVerification:\n- exact upstream position 4211: 2 passed, 0 failed/todo/unsupported/allowed-empty\n- contiguous positions 4202-4211: 10 files, 61 passed, all non-pass buckets zero\n- permanent focused regression suite: passed\n- zig fmt --check and git diff --check: passed\n\nContinues #397 and advances #66.