Lightly applies ESLint to OZTreeModule and removes demonstrably unused imports and straightforward unused variables

[ScaleNature]

Lightly applies ESLint to OZTreeModule and removes demonstrably unused imports and straightforward unused variables
The changes are intentionally conservative:

• removes demonstrably unused imports
• removes unused variables where the changes are mechanical and behavior-preserving
• slightly expands a small number of tests where previously unused variables became meaningful test assertions
• makes no intentional behavioral changes
• performs no refactoring beyond what is required for the cleanup

Some remaining no-unused-vars findings were intentionally left unchanged because they require project-level judgment (for example, complete theme palettes or interface methods) rather than mechanical cleanup. Whether those should be removed, suppressed, or retained is outside the scope of this PR.

Removing stale imports and straightforward unused variables makes module dependency declarations more accurately reflect the code they use and reduces maintenance noise.

ESLint is not part of build, test, or CI processes.
To run: npx eslint OZprivate/rawJS/OZTreeModule

The full test suite passes after these changes.

[lentinj]

This is a very worthwhile cleanup, but I'd rather that the unused imports were found by a linting tool, eslint / xo or similar. As well as having a bit more confidence in their work, it'd be something we can plug into the CI pipeline to stop unused imports cropping up in future.

OneZoom's code style is somewhat free range, so we'd have to have a very minimal set of rules at least to start with.

[ScaleNature]

Remaining unused variables found in OZTreeModule: npx eslint OZprivate/rawJS/OZTreeModule

src/factory/garbage_collection.js
74:10 error 'calc_real_richness' is defined but never used no-unused-vars

src/factory/polytomy_midnode.js
48:10 error 'display_cut_substr' is defined but never used no-unused-vars

src/themes/IUCN_explicit_colour_blind_friendly.js
4:5 error 'very_dark_grey' is assigned a value but never used no-unused-vars

src/themes/IUCN_explicit_theme.js
4:5 error 'very_dark_grey' is assigned a value but never used no-unused-vars

src/themes/genetic_conservation_theme.js
4:5 error 'very_dark_grey' is assigned a value but never used no-unused-vars
7:5 error 'mid_grey' is assigned a value but never used no-unused-vars
9:5 error 'very_light_grey' is assigned a value but never used no-unused-vars
17:5 error 'very_transparent_white' is assigned a value but never used no-unused-vars
23:5 error 'dark_blue' is assigned a value but never used no-unused-vars
28:5 error 'dark_orange' is assigned a value but never used no-unused-vars
29:5 error 'orange' is assigned a value but never used no-unused-vars
30:5 error 'light_orange' is assigned a value but never used no-unused-vars
48:10 error 'outline_highlight' is defined but never used no-unused-vars
88:10 error 'sponsor_color' is defined but never used no-unused-vars
96:10 error 'copyright_highlight_fill' is defined but never used no-unused-vars
104:10 error 'copyright_highlight_stroke' is defined but never used no-unused-vars
136:10 error 'copyright_text_highlight_fill' is defined but never used no-unused-vars

src/themes/natural_theme.js
4:5 error 'very_dark_grey' is assigned a value but never used no-unused-vars
6:5 error 'grey' is assigned a value but never used no-unused-vars
7:5 error 'mid_grey' is assigned a value but never used no-unused-vars
9:5 error 'very_light_grey' is assigned a value but never used no-unused-vars
17:5 error 'very_transparent_white' is assigned a value but never used no-unused-vars
74:10 error 'copyright_highlight_fill' is defined but never used no-unused-vars
82:10 error 'copyright_highlight_stroke' is defined but never used no-unused-vars
114:10 error 'copyright_text_highlight_fill' is defined but never used no-unused-vars

src/themes/popularity_colour_blind_friendly.js
4:5 error 'very_dark_grey' is assigned a value but never used no-unused-vars
16:5 error 'very_transparent_white' is assigned a value but never used no-unused-vars
18:5 error 'dark_blue' is assigned a value but never used no-unused-vars
19:5 error 'blue' is assigned a value but never used no-unused-vars
20:5 error 'light_blue' is assigned a value but never used no-unused-vars
22:5 error 'dark_red' is assigned a value but never used no-unused-vars
23:5 error 'red' is assigned a value but never used no-unused-vars
24:5 error 'light_red' is assigned a value but never used no-unused-vars
27:5 error 'pastel_green' is assigned a value but never used no-unused-vars
29:5 error 'pastel_cyan' is assigned a value but never used no-unused-vars
30:5 error 'pastel_magenta' is assigned a value but never used no-unused-vars
31:5 error 'pastel_yellow' is assigned a value but never used no-unused-vars

src/themes/popularity_theme.js
4:5 error 'very_dark_grey' is assigned a value but never used no-unused-vars
16:5 error 'very_transparent_white' is assigned a value but never used no-unused-vars
18:5 error 'dark_blue' is assigned a value but never used no-unused-vars
19:5 error 'blue' is assigned a value but never used no-unused-vars
20:5 error 'light_blue' is assigned a value but never used no-unused-vars
22:5 error 'dark_red' is assigned a value but never used no-unused-vars
23:5 error 'red' is assigned a value but never used no-unused-vars
24:5 error 'light_red' is assigned a value but never used no-unused-vars

[lentinj]

Thanks! After a quick look through I can't remember any gotchas about the lines in question, I think it's all just excessive copy-paste. Is there a reason you left the final unused variables, or just run out of time?

I'll have a bit more of a test then merge later in the week. Will have a look the final cases too in your comment.