feat: upgrade backend from zig 0.15 to 0.16
- smp_allocator replaces GeneralPurposeAllocator (removed in 0.16)
- std.Io.Threaded backend for async I/O (networking, http)
- std.Io.net replaces std.net for TCP server
- thread-per-connection replaces Thread.Pool (removed)
- compat.zig wraps removed APIs: getenv, sleep, timestamp, Mutex
- C file I/O replaces std.fs (removed) in timing.zig, LocalDb.zig
- ArrayList .empty replaces .{} init syntax
- all deps updated to 0.16-compatible hashes
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>