Bound plotting memory with explicit downsampling #13

Core recording and analysis are disk-backed in Datary 0.2.0, but Matplotlib plotting still materializes the selected records.

Add an explicit, deterministic downsampling policy for very large plots. Reports must state the original point count, plotted point count, algorithm, and parameters; missing-data markers and extrema should not disappear silently. Add memory and visual-regression coverage without changing the canonical records.