Add chdb vs pandas peak-memory benchmark

[wudidapaopao]

Self-contained benchmark that auto-generates test data (default 10M rows) and compares chdb SQL-pushdown vs pandas across 10 scenarios including filter, groupby, join, window functions, quantiles, and time-series. Measures peak memory via VmHWM (Linux) or ru_maxrss (macOS) in isolated subprocesses.

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