docs: document benchmark results (RQ1–RQ3) in README

[jathavaan]

What

Adds a ## Results section to the README summarizing the headline findings from the accompanying thesis (chapter 6), with one figure per research question and matching table-of-contents entries.

Findings surfaced

• RQ1 — single-machine: PostGIS wins wall-clock time in every cell; DuckDB trails by orders of magnitude; cost winner is mixed. → results-single-machine-wall-clock-time.png
• RQ2 — national-scale join: Sedona broadcast scales and beats single-node from the medium tier; partitioned runs ~2 orders slower / ~30× costlier and OOMs at medium+large. → results-distributed-wall-clock-vs-workers.png
• RQ3 — consistency: no configuration wins everywhere; winners flip across pattern and dimension. → results-winners-matrix.png

Notes

• Figures exported from the thesis PDFs to PNG (GitHub markdown does not render PDF inline) into .github/docs/img/.
• Run-time uses the minimum estimator, transferred bytes the median, each with a 95% bootstrap CI; full per-cell statistics remain in the thesis.

Closes #367