Zig 0.16 compatibility: std.process.args and std.time.Timer removed

Problem

The benchmark library doesn't compile under Zig 0.16 due to two std API changes:

1. std.process.args() → removed; args are now passed via std.process.Init to main(init: std.process.Init)
2. std.time.Timer → removed/restructured

src/benchmark_main.zig uses std.process.args() and src/benchmark.zig uses std.time.Timer.

Impact

ztls can't build zig build bench-bin under Zig 0.16. This blocks perf captures under 0.16 (ztls #62).

Fix needed

• benchmark_main.zig: conditionally accept std.process.Init under 0.16 (via `@hasDecl(std.process, "Init"))
• benchmark.zig: replace std.time.Timer with the 0.16 equivalent or a dual-version shim
• Options.parse / nextFlagValue / shortFlagValue: change *std.process.ArgIterator to anytype (both ArgIterator and Args.Iterator have .next() / .skip())

The ztls project pins this library at 6e7e9ab. Once fixed, update the hash in ztls build.zig.zon.

Start with src/benchmark_main.zig and src/benchmark.zig, then trace Options.parse, nextFlagValue, and shortFlagValue to understand the iterator interfaces. Build with Zig 0.16 using zig build bench-bin and verify the benchmark library remains compatible with the existing API; once it works, the ztls build.zig.zon hash can be updated.