Update Zig support and benchmark docs

[bytebrujo]

Summary

• update Zig native sources and build script for current Zig stdlib APIs
• add restored benchmark runner and fixture placeholder for focused Bun tests
• report Fast Mode separately in benchmark comparison and update landing page docs

Verification

• zig build
• zig build test
• bun test test/unit --timeout 30000
• bun run benchmark:compare
• landing build with bundled Node PATH
• browser render checks for landing desktop and mobile