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/parse.ts

@@ -853,12 +853,20 @@ function toLink(c: RawCell, labelOverride?: string): LinkSpec | null {
     if (!c.source || !c.target) return null
     const raw = (labelOverride ?? c.value).trim()
     const { label, step } = splitStep(raw)
+    const head = styleValue(c.style, "endArrow")
+    const tail = styleValue(c.style, "startArrow")
     return {
         id: c.id,
         source: c.source,
         target: c.target,
         label: label || undefined,
         dashed: styleValue(c.style, "dashed") === "1" || undefined,
+        bold:
+            Number(styleValue(c.style, "strokeWidth") ?? "1") >= 3 || undefined,
+        head,
+        tail,
+        headFill: head ? styleValue(c.style, "endFill") === "1" : undefined,
+        tailFill: tail ? styleValue(c.style, "startFill") === "1" : undefined,
         step,
         style: c.style,
     }