bench(compaction): add fusio-level IO probes for compaction/read amplification

[belveryin]

Summary

Instrument benchmark FS path with counting probes to capture operation counts and byte-level IO for compaction/read scenarios.

Motivation

Compaction performance changes are often driven by IO pattern shifts; we need direct counts, not just latency.

Scope

• Introduce benchmark-only counting FS wrapper (similar to existing test counting wrappers).
• Capture at minimum:
  • file read ops
  • file write ops
  • bytes read
  • bytes written
  • SST files touched
• Wire counters into benchmark JSON output.

Acceptance Criteria

1. Each scenario reports non-empty IO counters.
2. Counters are reset per scenario and isolated.
3. No invasive change to production FS interfaces.

Out of Scope

• Prometheus/OpenTelemetry export.

Labels

performance, benchmark, io, compaction

[linear]

DEV-523