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/graph.ts

@@ -49,6 +49,13 @@ export interface GraphEdge {
     target: string
     label?: string
     dashed?: boolean
+    /** Thick coloured arrow for THE key relationship. */
+    bold?: boolean
+    /** Arrowhead tokens, passed through — see LinkSpec. */
+    head?: string
+    tail?: string
+    headFill?: boolean
+    tailFill?: boolean
 }
 
 export interface GraphOptions {
@@ -356,6 +363,13 @@ export function graphToOperations(
             target: e.target,
             ...(e.label ? { label: e.label } : {}),
             ...(e.dashed ? { dashed: true } : {}),
+            ...(e.bold ? { bold: true } : {}),
+            ...(e.head !== undefined
+                ? { head: e.head, headFill: e.headFill ?? false }
+                : {}),
+            ...(e.tail !== undefined
+                ? { tail: e.tail, tailFill: e.tailFill ?? false }
+                : {}),
         })
 
     return {