docs(architecture): document shared-core/swappable-shell layering

Add a 'Layered separation' subsection to the Overview explaining that both
binaries render the same DashboardView driven by the same poll_loop, and only
the application shell differs (interactive main app vs display-only wallpaper
host). Note that the separate process is a correctness requirement (WorkerW
child lifecycle), and flag the remaining runtime-glue duplication tracked in
issue #135.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>