nutrientMainToolbarComposeView

Creates the ComposeView that hosts NutrientMainToolbar for coordinator.

This is the one piece of the old MainToolbarComposeBridge that must stay in Kotlin: a @Composable setContent { … } block can't be written from Java. The Java host (PdfUiImpl) calls this, adds the view to the toolbar coordinator layout, and pushes state through the coordinator's Java-friendly setMainToolbar* methods.

The view's id is R.id.pspdf__main_toolbar_compose so Espresso/UIAutomator can target the toolbar container, and the coordinator layout extends its slot over the status-bar strip.