Plots: refactor for stability, robustness, and expanded test coverage

[eliotmcintire]

Summary

• Stable filename generation using sim time (avoids timing-based instability)
• Error surfacing for failed base-R device saves (warning + failedFiles tracking)
• isBaseFormat variable to clarify base-R vs ggplot save path; funsUsed co-located with filenamesForSave in setup block; simplified useCache condition
• New tests covering: base-R plotting fn (hist), terra SpatRaster/SpatVector, named ... args without data =, ggplot object passed directly as data

Test plan

• All existing Plots tests pass (PASS 37 locally)
• New tests: base-R fn saves png + raw qs2, terra raster saves tif raw, terra vector saves qs2 raw, named ... args, direct ggplot data
• R CMD check locally
• GHA CI passes

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