websocket

0.16: migrate to std.Io — replace libc socket calls with Io vtable master

All networking now goes through std.Io vtable (netRead/netWrite/netClose) instead of raw libc recv/send/close. Mutex/Condition use Io variants. PriorityDequeue, DebugAllocator, and Io.Clock APIs updated for 0.16. All 31 tests pass. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>