fix for disk usage measurements

[MarkWolters]

This PR is a fix for an issue in how file sizes were calculated by the JVector test harness, i.e. BenchYAML and AutoBenchYAML.

Problem 1: The index cache directory accumulates across runs

In Grid.runOneGraph (and runAllAndCollectResults), BenchmarkDiagnostics monitors two directories:
diagnostics.startMonitoring("testDirectory", workDirectory);
diagnostics.startMonitoring("indexCache", Paths.get(indexCacheDir)); // "index_cache/"
getTotalBytes() sums both. The index_cache/ directory is persistent — it is never wiped between runs. So the reported disk figure grows monotonically across the run, regardless of which configuration is under test. The delta
between a fused-PQ run and a non-fused-PQ run is invisible.

Problem 2: Even within a single run, the snapshot captures all feature sets, not just one

When multiple feature sets are built in the same runOneGraph call, the directory total covers all of them. You can't attribute disk usage to a specific configuration from that number.

There is even a TODO in the code acknowledging this at line 233:
// TODO this does not capture disk usage for cached indexes. Need to update

---

The fix

Rather than directory-level monitoring, measure the specific graph file for each feature set directly after it is written. In buildOnDisk, the path for each feature set's graph file is already computed:

if (handles.containsKey(features)) {
graphPath = handles.get(features).writePath();
} else {
graphPath = outputDir.resolve("graph" + n++);
}

After the build completes, Files.size(graphPath) gives the exact on-disk size for that specific configuration. This value should be returned from buildOnDisk (as part of a result map keyed by feature set) and then included
in the BenchResult params or metrics map alongside the other per-configuration numbers.

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