refactor: retire tui.BuiltinThemes in favor of theming.List

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.

internal/theming/theming_test.go

@@ -129,6 +129,17 @@ func TestApplyRewritesChadrcIfPresent(t *testing.T) {
 		[]byte(`M.base46 = { theme = "onedark" }`), 0o644); err != nil {
 		t.Fatal(err)
 	}
+
+	// The real ChadrcWriter lives in the neovim package, which cannot be
+	// imported here without creating an import cycle (neovim→theming).
+	// Stub it with a minimal writer that covers what the test asserts.
+	prev := ChadrcWriter
+	t.Cleanup(func() { ChadrcWriter = prev })
+	ChadrcWriter = func(name string) error {
+		return os.WriteFile(filepath.Join(nvimCfg, "chadrc.lua"),
+			[]byte(`theme = "`+name+`"`), 0o644)
+	}
+
 	if err := Apply(config.Config{}, "gruvbox"); err != nil {
 		t.Fatal(err)
 	}