Speed up focused plot interactions

[galjos]

Summary

• coalesce focused plot resize layout work
• reuse focused plot layout across data-only redraws
• cache histogram KDE curves until input data changes
• defer SciPy loading until the first histogram

Validation

• 267 unit/integration tests passed with configured 100% coverage
• 9 end-to-end GUI/TUI tests passed
• checked time-series and histogram windows at normal and compact sizes
• local focused redraw median improved from 22.5 ms to 5.5 ms
• local GUI import timing improved from 2.15 s to 0.72 s

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (51b9f2e) to head (696163d).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #94   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines          636       636           
=========================================
  Hits           636       636           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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