NutrientMainToolbarNavigationIcon

The toolbar's own navigation (up / back) icon: com.pspdfkit.compose.toolbar.state.ToolbarFrame.navigationIcon, or nothing when the frame carries ToolbarIcon.None. NutrientMainToolbar's default navigationIcon, and the piece the SDK's own mount shows for PdfUi.setToolbarNavigationIcon.

Rendered as a standard toolbar item — the 48dp touch target, the theme's icon tint, the bar's ToolbarItemIndication, a long-press tooltip naming it — so it sits flush with the items. A tap dispatches ToolbarAction.NavigationClicked on coordinator; the item id it reports to accessibility and tests is R.id.pspdf__main_toolbar_navigation. Without a com.pspdfkit.compose.toolbar.state.ToolbarFrame.navigationContentDescription it is named "Navigate up" (AppCompat's localized abc_action_bar_up_description), like the View Toolbar's navigation button.

Public so a host composing its own slot can keep the SDK's affordance while adding to it, the way NutrientMainToolbarTitle is.