facett-syschart — a system-map chart: a handful of free-positioned
peer nodes (not a layered DAG) joined by edges, each carrying a badge
(a count / status number) and, when selected, an inline expandable detail
panel. Click a node to select it; its detail renders in a panel under the
canvas. The host owns what a node's detail is (a string here; richer hosts
draw their own widgets after ui() keyed off [SystemChart::selected]).
This is the piece [facett_core::draw]/[facett_graph::GraphView] don't
cover — those paint a non-interactive Scene; DepGraphView is a
layered DAG with a fixed deps/dependents drill-down. A "monitoring map" of
a few peer systems (a PKI/OIDC/Nexus triangle, a service mesh) wants
deterministic positions, a per-node badge, click-select, and a free-form
detail panel — that's [SystemChart].
Like every facett component it implements [Facet]: title / ui /
state_json (every node, badge, edge + the selection), so it drops into a
FacetDeck and is robot-testable through facett_core::harness — drive
[SystemChart::select] headlessly and assert state_json.