Stop editor defaults from leaking into highlight groups

[wadackel]

Summary

:hi merges into a group's existing attributes, so every key the generator omitted kept the editor's default instead of meaning "unset". After colorscheme dogrun:

• Vim 9.2: SpellBad/SpellCap/SpellLocal/SpellRare kept ctermbg=12/9/11/13 (bright red/blue backgrounds on misspelled words in 256-color terminals), Identifier stayed bold, Visual got guifg=LightGrey (syntax colors lost inside a selection), Underlined kept a blue fg, and several message/diff groups stayed bold.
• Neovim 0.12: Pmenu kept cterm=reverse and Visual kept ctermfg=0 (visible without termguicolors), MatchParen and DiffDelete stayed bold, and every Spell/Diagnostic underline used Neovim's own palette as its underline color.

highlight() now writes all seven keys (guifg ctermfg guibg ctermbg guisp gui cterm) and uses NONE where dogrun sets nothing, which is what the original hand-written colorscheme did.

Related changes:

• SpellCap and DiagnosticUnderline{Info,Hint,Ok} only had an underline and relied on the leaked defaults to be told apart; they now get a dogrun underline color (yellow / infofg) while keeping the text color.
• GitSignsChangeDelete / GitGutterChangeDelete used difftextbg, a background color (1.54:1 contrast on mainbg); they now use the Change color. NvimTreeGitDeletedIcon / FylerGitDeleted use pink.
• Operator was undefined, so legacy syntax in Neovim rendered operators in Neovim's default grey; it now matches @operator (purple).
• Vim's :terminal now gets the same 16 colors via g:terminal_ansi_colors (used by Vim with termguicolors or in the GUI).
• Generator cleanup: 16 unused palette entries removed, palette lookup failures name the color and the group, and integration tests run the binary Cargo built (CARGO_BIN_EXE_dogrun) instead of a nested cargo run.

term= attributes are intentionally left alone: dogrun defines none, so the editor defaults remain the only thing that works on monochrome terminals.

Verification

• just check (fmt, clippy, all tests including the golden byte-compare) passes; new tests assert every hi line carries all seven keys and that the Vim branch sets 16 ANSI colors.
• Loading the colorscheme and reading back nvim_get_hl() / hlget(): groups holding a value dogrun did not set went from 13 → 0 (Neovim) and 12 → 0 (Vim).
• Diffing every highlight group between the previous and the new colorscheme under a full Neovim config with plugins: only the intended groups change (plus GitSignsStagedChangedelete, which gitsigns derives from GitSignsChangedelete).

References

• n/a

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