Split a struct output into one pin per field (#256)

Reading one field of a struct needed its own Expand node, even where
the graph already knows the struct type. A split output shows the
fields directly on the node, so a Find … Match test becomes a single
Equals and the wires keep running left to right.

- Right-clicking an output pin offers "Split into <Type> fields" when
  the type is known and "Split as …" for every other struct asset.
  The output is replaced by one pin per field with the field's kind.
- The type is known for typed struct variables, typed Pack/Expand,
  fields of another split and item of For each, Find, Match and Item
  at over a typed list (portSplits.ts).
- Right-clicking a field pin offers "Recombine". Splitting drops the
  edges of the whole output, recombining drops the field edges. One
  undo step brings them back.
- The split is stored as GraphNode.splits and saved with the graph.
  Copy and paste carry it along.
- Field pins follow the struct asset like a typed Expand. A new field
  gets a pin, a removed field loses pin and edge, a renamed field keeps
  its edge. Deleting the asset keeps the last fields. The delete dialog
  counts splits, so it now says "node(s)" instead of "Pack/Expand
  node(s)".
- Renaming a variable carries its split and the field edges along.
- At run time the node still produces the whole value. A field pin
  reads its field like Expand, a missing field reads as empty.
- Limits: outputs only, one level, behavior graphs only.

Tests:
- ./check.sh: 185 files, 2706 tests, CHECK EXIT 0
- e2e GM_STEPS=1-6,54,55: ALL PASSED, 303 checks
- New port-splits.test.ts with 12 tests. e2e 6z-vi with 3 checks
  using a real mouse: split Find.item, wire item.Name into the test
  and Match.item.Amount into the log (logs 7), recombine.
- Counter-checks: without field reads in the interpreter, without
  rename in the split, without moving edges on rename and without
  cleanup on field removal one unit test fails each. In the browser,
  without field reads nothing is logged and without type inference
  the split entry is missing.
- Check total in README: 1520.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/editor/graph/GraphCanvas.ts

@@ -55,6 +55,7 @@ import { DRAG_THRESHOLD, GraphDrags } from './graphDrags';
 import type { GraphCanvasHost } from './graphHost';
 import { openAnnotationMenu, openLinkMenu, openNodeMenu } from './graphMenus';
 import { NodePalette, type PaletteAction } from './NodePalette';
+import { splitEntries } from './splitMenu';
 import { type PaletteEntry, paletteEntries } from './paletteEntries';
 import { NODE_WIDTH, NodeView, TITLE_HEIGHT } from './NodeView';
 import { drawGraphGrid, FIT_MARGIN, ZOOM_LIMITS, zoomedAbout } from './graphView';
@@ -490,7 +491,12 @@ export class GraphCanvas extends Widget {
       const { id } = view.node;
       if (hit) {
         const key = hit.port.key;
-        this.openLinks((g) => (hit.isOutput ? edgesOutOf(g, id, key) : edgesInto(g, id, key)), e.global);
+        const extras = hit.isOutput ? splitEntries(this.state, this.assetId, view.node, hit.port) : [];
+        this.openLinks(
+          (g) => (hit.isOutput ? edgesOutOf(g, id, key) : edgesInto(g, id, key)),
+          e.global,
+          extras,
+        );
       } else {
         // Right-clicking inside a multi-selection acts on all of it; outside it, on that node alone.
         if (!this.state.selectedNodeIds.includes(id)) this.state.select(nodeSelection(this.assetId, [id]));
@@ -743,9 +749,13 @@ export class GraphCanvas extends Widget {
     if (this.state.execute(command)) this.state.select(nodeSelection(this.assetId, [command.node.id]));
   }
 
-  private openLinks(pick: (graph: Graph) => Graph['edges'], at: { x: number; y: number }): void {
+  private openLinks(
+    pick: (graph: Graph) => Graph['edges'],
+    at: { x: number; y: number },
+    extras: readonly MenuItem[] = [],
+  ): void {
     const graph = this.asset()?.graph;
-    if (graph) openLinkMenu(this.state, this.assetId, pick(graph), at.x, at.y);
+    if (graph) openLinkMenu(this.state, this.assetId, pick(graph), at.x, at.y, extras);
   }
 
   /** Shortened to the canvas: since the Particles tab put a preview beside it, a canvas can be narrow. */