Edge selection lands as a parallel `selected_edges: Vec<EdgeId>` on
DiagramView with click hit-testing against the rendered polyline
(EDGE_HIT_RADIUS = 5 screen px), shift / ctrl-click semantics matching
shapes, Ctrl+A inclusion, delete with a new HistoryEntry::DeleteEdges
variant, and a wide blue selection chrome over the edge stroke.

Edge style mutators (set_edge_stroke_color / _width / _dashed /
_arrow_start / _arrow_end / _routing + cycle_edge_routing) record
EditEdgeStyle / EditEdgeRouting / EditEdgeLabel entries with full undo
/ redo. Stroke colour and width on the existing toolbar cells now flow
into edges when an edge is selected so the same gesture covers shapes
and edges.

Edge label editing reuses the PR7 overlay machinery. EditingState is
now an enum (Shape | Edge); F2 with a single edge selection opens a
TextEdit over the polyline midpoint via edge_label_edit_id, normalises
empty input to None on commit, and records EditEdgeLabel.

Four new toolbar cells (EdgeRouting, EdgeDashed, EdgeArrowStart,
EdgeArrowEnd) live in their own section, only enable when an edge is
selected, and reflect current edge state via cell_highlighted.

The colour swatches gain a real picker: a secondary-click opens egui's
RGB picker popup anchored under the swatch, applies colour changes as
they happen, and dismisses on Escape / outside-click / Close. The
existing left-click palette cycle stays for muscle memory and to keep
PR8's one-tap workflow intact.

18 new integration tests cover selection, hit-testing, history round
trips, toolbar dispatch, F2 label entry, and picker layout — including
a wider-screen helper because the toolbar grew past 800px with the new
edge section.

@@ -0,0 +575,4 @@
+/// Format an `f32` for the SVG attribute stream — three decimals,
+/// trailing zeros trimmed, so the output is compact without slipping
+/// into scientific notation on extreme values.
+fn fmt(v: f32) -> String {

@@ -5091,0 +5246,4 @@
+    });
+    view.cycle_edge_routing(&mut diagram);
+    assert!(matches!(
+        diagram.edges.iter().find(|e| e.id == edge_id).unwrap().routing,

@@ -0,0 +626,4 @@
+
+    let svg = to_svg(&d);
+    let opts = usvg::Options::default();
+    usvg::Tree::from_str(&svg, &opts).expect("complex svg parses");