Rewrite parts of theme61 to use S3 methods

[aaronw22]

To Do list

1. Introduce canonical plot coercion

• Add as_e61_plot() S3 generic
• Methods for:
  • ggplot
  • e61_plot (idempotent)
  • (optional) patchwork, list
• Ensure both theme61::ggplot() and save_e61() call as_e61_plot()

2. Make “map vs non-map” a first-class concept

• Introduce plot classes:

  • e61_plot
  • e61_map (inherits from e61_plot)

• Decide classification strategy:

  • explicit (as_e61_map()), or
  • inferred once at save/preview time

• Remove repeated if (is_spatial(...)) logic where possible

3. Centralise save/preview branching with internal generics

• Add internal generic(s), e.g.:
  • e61_prepare(plot, ...)
  • e61_plot_dims(plot, ...)
• Implement methods:
  • e61_prepare.e61_plot
  • e61_prepare.e61_map
• Refactor save_e61(), save_single(), save_multi() to call these generics instead of branching inline

4. Convert ggplot helpers into spec objects using ggplot_add()

• Refactor helpers such as:
  • plot_label()
  • add_baseline()
  • add_e61_logo()
  • geom_pointbar()
  • aes_y_lims(), aes_labs(), labs_e61()
• Each helper returns a lightweight spec object
• Implement ggplot_add.<spec_class>() methods
• Allow behaviour to depend on e61_plot vs e61_map

5. Unify theme_e61() and spatial theme logic

• Make theme_e61() return an e61_theme spec object
• Implement ggplot_add.e61_theme() that:
  • applies regular theme for e61_plot
  • applies spatial theme for e61_map
• Optionally keep theme_e61_spatial() as a thin wrapper that just tags the plot as a map

6. Refactor y-scale and secondary axis logic into S3 specs

• Create spec classes (e.g. e61_y_scale, e61_secondary_axis)
• Move complex branching out of:
  • scales.R
  • sec_axes_rescale.R
• Handle map vs non-map differences via dispatch instead of conditionals

7. Return structured results from save_e61()

• Introduce an e61_save_result class
• Include metadata (paths, dimensions, device, preview info)
• Add methods:
  • print.e61_save_result()
  • as.character.e61_save_result()
  • (optional) knit_print.e61_save_result()

Describe your changes

Do the following before requesting a review

For feature branches

• I have written tests covering functions I have added or changed.
• I have run tests and they all pass.
• I have ensured all new functions show up in the _pkgdown.yml file.
• I have updated the package documentation with devtools::document().
• I have updated DESCRIPTION with any new package dependencies.

Additional steps for the last PR into dev prior to releasing a new version from dev to main

• I have updated DESCRIPTION with the new package version number.
• I have updated NEWS.md with a brief description of my changes.
• I have ensured the _pkgdown.yml file is correctly built by running pkgdown::check_pkgdown().
• I have run devtools::build_readme() to update README.md.
• I have updated the package website with pkgdown::build_site_github_pages().