Adds the measurement core:
- CountingAllocator: a thin wrapper over std.mem.Allocator vtable that
tracks alloc/free counts and total bytes allocated, exposing a reset()
used between attempts of the adaptive loop.
- stats.summarize: min / mean / sample stddev over an array of f64,
used to aggregate --count repetitions.
- Benchmark: user-facing handle passed to each bench fn — n,
allocator (counting-wrapped), io, with snake_case methods
reset_timer / stop_timer / start_timer / set_bytes / report_allocs /
keep / run (sub-bench). Times via std.Io.Timestamp.now(io, .awake).
- runner.run_one: galloping iteration count toward --min-time, with
×100 growth cap per step and "nice-number" rounding, matching Go's
testing pkg heuristic.
Initial Zig 0.16 build script wiring the public `zbench` module, the
`zbench_build` helper module for downstream `build.zig` files, an
internal `test` step, and an `example` step that compiles and runs the
bundled benchmark suite.
