After the egui_plot 0.38 upgrade the PlotMemory id derived with
Id::unique("scope_plot_<name>") no longer matched the id Plot::show uses
internally (id_salt is now an IdSalt, not an Id), so PlotMemory::load
returned None and the legend overlay was never rendered. The memory id
is now computed once and pinned explicitly via Plot::id(...), which also
restores legend click-to-hide, isolate interactions, hidden-item sync
and the FFT panel's hidden-trace readback.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>