Make internal/theming.List the single source of truth for themes:
- Drop tui.BuiltinThemes, tui.ThemeByID, and the NvChadTheme field on
tui.Theme. tui.Theme is now a thin {ID, Name} display value.
- Wizard populates its dropdown from theming.List() and uses
DisplayName for the label.
- neovim.resolvedTheme uses theming.Get (with theming.List()[0] as a
fallback) instead of a caller-supplied []tui.Theme slice.
- neovim.New signature loses the themes []tui.Theme parameter; update
cmd/archduino/main.go and all neovim tests to match.
Break the resulting neovim→theming import cycle (theming previously
imported neovim for WriteChadrc) by inverting the dependency:
theming exposes a ChadrcWriter function hook, and neovim registers
its WriteChadrc into it via package init. The theming test stubs
ChadrcWriter directly to exercise the chadrc-rewrite branch without
pulling neovim in and re-creating the cycle.
Delete the now-obsolete TestBuiltinThemesCarryNvChadName / TestThemeByID*
tests in internal/tui/state_test.go — their coverage lives in the
theming package's own tests.
