Restore distinct linetypes in plot.owsa black-and-white mode

[jurjoroa]

Goal

plot.owsa(col = "bw") maps linetype through aes_(), which eagerly evaluates its arguments in the function environment where strategy <- NULL was set to silence R CMD check. linetype therefore maps to NULL rather than the data column, so every strategy receives the same linetype and collapses into one indistinguishable line on a documented, vignette-used argument.

Scope

• Replace aes_(linetype = strategy) with aes(linetype = strategy) at R/owsa.R:129
• Switch the deprecated size argument to linewidth in the same call
• Add a regression test asserting one distinct linetype and one group per strategy in the bw branch
• Confirm the col = "full" branch is unchanged

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Version: #172