Implement JVM GC management to reduce measurement noise (issue #134)

[Copilot]

GC pauses introduce noise spikes into sample measurements. This adds two mitigations: pre-benchmark GC triggering to drain heap pressure before I/O starts, and per-sample GC detection with automatic retake on detection.

New: GcManager

Utility class backed by the standard ManagementFactory.getGarbageCollectorMXBeans() API (no internal APIs):

• triggerPreBenchmarkGc() — calls System.gc() before benchmark execution begins
• getTotalGcCount() / gcOccurredSince(snapshot) — lightweight GC event detection via collection count delta

long gcCountBefore = GcManager.getTotalGcCount();
action.perform(sample);
boolean gcDetected = GcManager.gcOccurredSince(gcCountBefore);

BenchmarkRunner changes

• execute(): calls GcManager.triggerPreBenchmarkGc() after recordStartTime(), before any I/O phase
• runOperation(): wraps each sample in a do-while retry loop; on GC detection, undoes the per-block progress counter increments (via LongAdder.add(-numBlocks)) and retakes the sample up to MAX_GC_RETRIES = 3 times, logging each retake with retry index

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[jamesmarkchan]

PR #143 looks more through so will use that.