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 {

lib/diagram-engine/operations.ts

@@ -296,6 +296,12 @@ export const OperationSchema = z.discriminatedUnion("op", [
     }),
     z.object({
         op: z.literal("link"),
+        id: z
+            .string()
+            .optional()
+            .describe(
+                "Edge id. Required for a second edge between the same two nodes (parallel relationships), so each can be addressed later",
+            ),
         source: z.string(),
         target: z.string(),
         label: z.string().optional(),
@@ -303,6 +309,31 @@ export const OperationSchema = z.discriminatedUnion("op", [
             .boolean()
             .optional()
             .describe("Dashed line — replication, sync, policy"),
+        bold: z
+            .boolean()
+            .optional()
+            .describe(
+                "A thick coloured arrow for THE key relationship — a transformation, the main flow. Use sparingly: one or two per diagram",
+            ),
+        head: z
+            .string()
+            .optional()
+            .describe(
+                "Arrowhead at the target. block/open/diamond/diamondThin/oval/cross/none, ER: ERone/ERmany/ERoneToMany/ERzeroToMany/ERzeroToOne. UML inheritance: head=block headFill=false. Omit for a plain arrow",
+            ),
+        tail: z
+            .string()
+            .optional()
+            .describe(
+                "Arrowhead at the source, same values as head. UML composition: tail=diamondThin tailFill=true. ER 1:N: tail=ERone head=ERoneToMany",
+            ),
+        headFill: z
+            .boolean()
+            .optional()
+            .describe(
+                "Fill the head. Meaning-bearing in UML: filled diamond=composition, hollow=aggregation",
+            ),
+        tailFill: z.boolean().optional(),
         step: z
             .number()
             .optional()
@@ -685,25 +716,52 @@ export function applyOperations(
                     errors.push(`link: no node with id "${op.target}"`)
                     break
                 }
-                // A second arrow between the same pair is normally a mistake — two identical
-                // lines drawn on top of each other — EXCEPT between two participants of a
-                // sequence diagram, where a back-and-forth conversation is the whole point.
-                // There the messages are distinguished by their step, not by their endpoints.
+                // A second arrow between the same pair WITHOUT an id is a mistake — two
+                // identical lines on top of each other. With an id it is a parallel
+                // relationship (an ER diagram's "places" and "cancels" between the same
+                // two entities), addressable separately. Sequence messages are exempt
+                // as before: their identity is the step, not the endpoints.
                 const conversation = sameSequence(tree, op.source, op.target)
                 const dup =
                     !conversation &&
+                    !op.id &&
                     tree.links.some(
                         (l) => l.source === op.source && l.target === op.target,
                     )
                 if (dup) {
                     errors.push(
-                        `link: "${op.source}" → "${op.target}" already exists`,
+                        `link: "${op.source}" → "${op.target}" already exists — give this one an id to draw a second, parallel relationship`,
+                    )
+                    break
+                }
+                if (op.id && tree.links.some((l) => l.id === op.id)) {
+                    errors.push(`link: edge id "${op.id}" is already taken`)
+                    break
+                }
+                // Arrowhead tokens reach the style string; the same charset gate as
+                // shapes keeps `block;dashed=1` from smuggling style keys in.
+                const badHead = [op.head, op.tail].find(
+                    (v) => v !== undefined && !/^[a-zA-Z0-9]+$/.test(v),
+                )
+                if (badHead !== undefined) {
+                    errors.push(
+                        `link: arrowhead "${badHead}" contains characters that are not allowed`,
                     )
                     break
                 }
                 const link: LinkSpec = { source: op.source, target: op.target }
+                if (op.id) link.id = op.id
                 if (op.label) link.label = op.label
                 if (op.dashed) link.dashed = true
+                if (op.bold) link.bold = true
+                if (op.head !== undefined) {
+                    link.head = op.head
+                    link.headFill = op.headFill ?? false
+                }
+                if (op.tail !== undefined) {
+                    link.tail = op.tail
+                    link.tailFill = op.tailFill ?? false
+                }
                 if (op.step != null) link.step = op.step
                 tree.links.push(link)
                 break

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,
     }

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;"

lib/diagram-engine/types.ts

@@ -244,6 +244,22 @@ export interface LinkSpec {
     label?: string
     /** Dashed line — replication, sync, policy, lineage. */
     dashed?: boolean
+    /**
+     * A bold arrow: the relationship IS the point — a transformation, the main flow.
+     * Thick and coloured, a visual element rather than a hairline connector.
+     */
+    bold?: boolean
+    /**
+     * Arrowhead at the target / at the source. draw.io endArrow/startArrow tokens:
+     * block, open, diamond, diamondThin, oval, cross, ERone, ERmany, ERoneToMany,
+     * ERzeroToMany, ERzeroToOne, none… Unset means the default (classic at the target,
+     * nothing at the source). `headFill`/`tailFill` distinguish UML composition
+     * (filled diamond) from aggregation (hollow) — conventions where fill IS meaning.
+     */
+    head?: string
+    tail?: string
+    headFill?: boolean
+    tailFill?: boolean
     /** Step number, rendered as an "N. " prefix on the label. */
     step?: number
     /** Verbatim style, when recovered from XML. */