[codex] Add length axis option to size plots

[gustavdelius]

Summary

Adds size_axis = c("w", "l") to size-based plotting APIs so users can show length in cm instead of weight in grams on the size axis. Length values are derived from the species-specific allometric weight-length relationship via the existing w2l() helper.

Details

• Keeps size_axis = "w" as the default to preserve existing plot behaviour.
• Applies length axes to spectra, CDF, feeding level, predation mortality, fishing mortality, diet, generic size-array plots, comparison helpers, plotly wrappers, and animations.
• Omits Resource and Total curves for length-axis plots because they do not have a species-specific weight-length relationship.
• Updates return_data = TRUE output so the plotted size column is l when size_axis = "l".
• Updates documentation, NEWS, and focused plot tests.

Validation

• devtools::load_all(); testthat::test_file("tests/testthat/test-plots.R")
• Smoke-tested plotlySpectra(..., size_axis = "l") and animate(..., size_axis = "l").

[gustavdelius]

Fixed the example-check failure from plot2(getBiomass(...), getBiomass(...)).

Root cause: the shared comparison helpers were defaulting to size-axis conversion and assumed a w column, even for time-series comparisons whose x column is Year. They now only perform w/l conversion when a caller explicitly passes size_axis.

Validation rerun:

• devtools::load_all(); testthat::test_file("tests/testthat/test-plots.R")
• Exact failing example commands from plot2 documentation now print without error.

[gustavdelius]

Fixed the Rd warning for plotSpectraRelative.Rd: size_axis is now documented in the roxygen block and the Rd file has been regenerated.

Updated branch head: 07266b85.