feat(diagram-engine): connection vocabulary — arrowheads, parallel edges, edge ids

Arrowheads carry meaning: a crow's foot IS one-to-many, a hollow diamond IS
aggregation. The engine allowed exactly one arrowhead; this opens the
vocabulary the same way shapes were opened:

- LinkSpec gains head/tail (pass-through to endArrow/startArrow, charset-
  gated against style injection) and headFill/tailFill — fill is written
  explicitly whenever a head is declared, because UML composition and
  aggregation differ ONLY by fill and draw.io's per-head default would flip
  the meaning. bold (4px amber, for THE key relationship) included.
- Parallel edges: a second link between the same pair is allowed when it
  carries an id (ER's 'places' and 'cancels' between the same two entities);
  without one it stays an error, since two identical overlapping lines is a
  mistake. Edge ids are also what later operations address.
- Sequence messages respect a declared head (an async message's open arrow
  is UML notation) while defaulting to the solid block as before.
- draw_graph's edge schema extended to match; GraphEdge passes the new
  fields through to the link operations it generates.

5 new tests: crow's foot style emission, hollow-vs-filled round trip,
injection rejection, parallel-edge gating, bold round trip. 561 unit tests
green. ER+UML acceptance diagram (crow's foot, zero-to-one, hollow
inheritance triangle, filled composition diamond) verified in the real
editor.

lib/diagram-engine/render.ts

@@ -590,6 +590,18 @@ function edgeXml(
     let style = l.style ?? EDGE_STYLE
     if (!l.style) {
         if (l.dashed) style += "dashed=1;"
+        // A bold link is a visual element, not a connector: thick amber with a filled
+        // block head — the "this becomes that" arrow of a comparison.
+        if (l.bold)
+            style +=
+                "strokeWidth=4;strokeColor=#D79B00;endArrow=block;endFill=1;endSize=6;"
+        // Arrowhead vocabulary, passed through to draw.io. Fill is written whenever
+        // the head is: UML composition vs aggregation differ ONLY by fill, so leaving
+        // it to draw.io's per-head default would flip the meaning.
+        if (l.head !== undefined)
+            style += `endArrow=${l.head};endFill=${l.headFill ? 1 : 0};`
+        if (l.tail !== undefined)
+            style += `startArrow=${l.tail};startFill=${l.tailFill ? 1 : 0};`
         if (label) style += "labelBackgroundColor=light-dark(#FFFFFF,#0B0F14);"
     }
     if (route)
@@ -646,7 +658,14 @@ function messageXml(
     const self = l.source === l.target
     let style = l.style ?? EDGE_STYLE
     if (!l.style) {
-        style += "endArrow=block;endFill=1;html=1;"
+        // The declared head wins over the sequence default: an async message drawn
+        // with an open arrow is UML notation, not decoration.
+        style +=
+            l.head !== undefined
+                ? `endArrow=${l.head};endFill=${l.headFill ? 1 : 0};html=1;`
+                : "endArrow=block;endFill=1;html=1;"
+        if (l.tail !== undefined)
+            style += `startArrow=${l.tail};startFill=${l.tailFill ? 1 : 0};`
         if (l.dashed) style += "dashed=1;"
         style += "labelBackgroundColor=light-dark(#FFFFFF,#0B0F14);"
         style += self ? "edgeStyle=orthogonalEdgeStyle;" : "edgeStyle=none;"