Monitor and plot RSS memory and CPU usage during qlever index

[tanmay-9]

So far, the qlever index command gave no insight into how much memory an index build actually needs or which index phase is responsible for the peak.

With this change, every index build records RSS memory and CPU usage over time, writes <name>.usage-log.tsv, and renders <name>.usage-log.png once the index build finishes. The plot shades each index build phase (parsing, vocabulary merge, conversion, each permutation, and the text index) as a separate band and annotates the memory peak, so resource usage can be attributed to a specific phase. For comparison across runs and settings, the plot is captioned with the git hash of the index binary, the STXXL_MEMORY setting, and the batch size. This works whether the index is built natively or in a container (docker / podman).

The sampling rate can be set with --resource-monitor-interval and the plot density on long builds with --resource monitor-plot-max-points (the sampling itself is unaffected, only how many points are drawn). There is also a --replot-resource-usage option that re-renders the plot from an existing <name>.usage-log.tsv without re-running the index build, which is useful for tweaking plot settings.