Return a valid icers object from single-strategy calculate_icers

[jurjoroa]

Goal

calculate_icers()'s nstrat == 1 early return at R/icers.R:80-83 skips the Status column, the column ordering and the class assignment, so it hands back a bare data.frame instead of an icers object. plot() then dispatches to plot.data.frame and fails with need finite 'ylim' values.

Scope

• At R/icers.R:80-83, set Status to "ND", assign Inc_Cost/Inc_Effect/ICER as NA, reorder to the same 7-column layout as the multi-strategy return, and set class(df) <- c("icers", "data.frame")
• Add a regression test asserting the single-strategy return has class icers and the same columns in the same order as a multi-strategy return
• Add a regression test asserting plot() on a single-strategy result returns a ggplot object
• Coordinate with Guard calculate_icers empty input and remove stale roxygen #178, which adds the nstrat == 0 guard in the same block

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Version: #172