ui::screen has no way to mark part of its document as shell chrome, separate from the page body #1027

Problem

A screen's builder (`Screen::Builder = std::function<Node(Scope&)>`) returns one `Node` — the whole document mounts as a single tree in one place (via `DocumentScreenHost`/`mountInto`). There is no way for a screen to say "this part is shell chrome (a title bar with primary actions), this part is the scrollable page body" so a client's shell can lay the two out differently — e.g. a full-bleed, unpadded toolbar strip above a padded, scrollable body, which is a common app-shell pattern (also the Lastrada Design System's own shell: a white `.top` title bar flush with the window edges, above a padded `.body`).

Today, a screen that wants a distinct "title bar" row can only tag it with `.surface("toolbar")` and nest it as the first child of its own content tree. Since the whole tree mounts into one client-side container, that row is subject to whatever padding the shell's own content area applies uniformly — it can never reach the window's actual edges the way a true app-shell title bar does, no matter how it's styled. `ContainerWidget` also has no cross-container reparenting API (`moveChild` only reorders within the same parent), so a client can't pull a tagged sub-tree out after mounting either.

Suggested direction

Something at the `ui::screen`/document level that lets a screen declare two logical regions — chrome and body — which a `DocumentScreenHost`-based client can mount into two different widgets (e.g. the existing `ShellFrame`'s `topBar`/`page`, or a custom shell's equivalent slots), rather than being forced to interpret document structure/surface names after the fact. Even a minimal form — e.g. `ui::screen` accepting an optional second builder for chrome content, surfaced to the host as a second named slot in `Emitted` — would unblock this.

Context

Hit while matching a licence-management client's Qt Quick GUI to its design: every screen's title + primary actions row is tagged `.surface("toolbar")` already (confirmed the backend scheme/background-rectangle plumbing for that surface name works correctly, down to `MorphWidget.qml`'s generic background `Rectangle`), but the row can only ever render as a padded, inset rectangle — never the edge-to-edge bar the design calls for — because the client's shell has no way to mount it outside the body's own padding.