Add arbitrary BenchResult metadata and explicit parse control, replacing
the previous code/elapsed fields. Make BenchResult subscriptable by
subbenchmark name and make SubBenchResult list-like over its states.
Extend SubBenchState parsing to expose summaries by tag, read paired
sample frequency data, return None for unavailable sample/frequency
files, and validate matching sample/frequency lengths.
Harden parsing for NVBench JSON output with no-axis benchmarks, null
axis_values, skipped states with null summaries, float axis input_string
lookups, and recorded sidecar binary paths.
Expand BenchResult tests to cover metadata, parse=False, sequence-style
access, frequency-aware centers, missing binary data, skipped states,
and mismatched sample/frequency counts.
Example usage:
```
import array, numpy as np, cuda.bench
r = cuda.bench.BenchResult("perf_data/axes_run1.json")
r["copy_sweep_grid_shape"].centers_with_frequencies(
lambda t, f: np.median(np.asarray(t)*np.asarray(f)))
```
