A graph that grew past one screen needs somewhere to say what a part of it is for (#38)

The graph could hold any number of nodes and said nothing about which of them
belonged together. Groups and comments are two separate things, as decided in
the issue: a group is a titled box that carries the nodes it encloses, a
comment is free text that carries nothing.

Both live as optional arrays on Graph, not as registry nodes. A comment that
were a real node would run through the interpreter, show up in the drop
palettes and be compiled into a shader — the wrong place for a piece of text.
As arrays on Graph they exist in all four graph kinds at once, and a document
without them means exactly "no groups", so no format bump.

Membership is computed when the box is grabbed, never stored: the center of a
node inside the box travels with it, and a node dragged out is simply out. A
box inside a box travels with the outer one, its own nodes included. A group
is grabbed by its title bar only — its body stays transparent to the pointer,
or a rubber band could never be drawn across the nodes it holds. Deleting a
box deletes the box; the nodes stay where they are, because a box that
silently took ten nodes with it would be the most expensive surprise in the
feature.

The color is a free picker, and the title color follows from it: relative
luminance per WCAG, threshold at 0.179 where the two contrast ratios meet —
not 0.5, which would pick white on mid grey where black reads better.

Resizing snaps the far edge to the grid, not the width, so the corner that is
not being dragged stays put. The minimum size rises in whole steps for that
reason: 80 is no multiple of 24, so a box squeezed to the stop would have sat
off-grid in exactly the mode meant to keep it on.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/graph/Graph.ts

@@ -57,15 +57,71 @@ export interface GraphEdge {
   label?: string;
 }
 
+/**
+ * A box drawn behind the nodes, carrying a line of text and a colour. Two kinds share this shape
+ * because they differ in what they *mean*, not in what they hold: a group frames the nodes whose
+ * centre lies in it and takes them along when moved, a comment frames nothing and is read.
+ * Purely visual either way – no interpreter ever looks at one.
+ */
+export interface GraphAnnotation {
+  id: string;
+  x: number;
+  y: number;
+  w: number;
+  h: number;
+  /** A group's title line; a comment's body, which wraps. */
+  text: string;
+  /** 0xRRGGBB. Absent means the theme's default, so a project never stores a colour nobody chose. */
+  color?: number;
+}
+
+/** Which of the two lists an annotation lives in – the only thing that tells them apart. */
+export type AnnotationKind = 'group' | 'note';
+
+/** Smallest box either kind may be dragged down to; below this the text has nowhere to go. */
+export const MIN_ANNOTATION_SIZE = { w: 80, h: 48 };
+
 export interface Graph {
   nodes: GraphNode[];
   edges: GraphEdge[];
+  /** Titled boxes that carry what they enclose. Absent means none – an old document says exactly that. */
+  groups?: GraphAnnotation[];
+  notes?: GraphAnnotation[];
 }
 
 export function createEmptyGraph(): Graph {
   return { nodes: [], edges: [] };
 }
 
+/** The list of one kind, created on demand: a graph without annotations stores neither array. */
+export function annotationList(graph: Graph, kind: AnnotationKind): GraphAnnotation[] {
+  const list = kind === 'group' ? graph.groups : graph.notes;
+  if (list) return list;
+  const fresh: GraphAnnotation[] = [];
+  if (kind === 'group') graph.groups = fresh;
+  else graph.notes = fresh;
+  return fresh;
+}
+
+/** Reads without creating, so drawing and lookups leave an untouched graph untouched. */
+export function annotationsOf(graph: Graph, kind: AnnotationKind): readonly GraphAnnotation[] {
+  return (kind === 'group' ? graph.groups : graph.notes) ?? [];
+}
+
+/** An annotation by id, whichever list holds it. Ids are unique across both. */
+export function findAnnotation(
+  graph: Graph,
+  id: string,
+): { kind: AnnotationKind; annotation: GraphAnnotation } | null {
+  for (const kind of ANNOTATION_KINDS) {
+    const annotation = annotationsOf(graph, kind).find((a) => a.id === id);
+    if (annotation) return { kind, annotation };
+  }
+  return null;
+}
+
+export const ANNOTATION_KINDS: readonly AnnotationKind[] = ['group', 'note'];
+
 export function findNode(graph: Graph, nodeId: string): GraphNode | undefined {
   return graph.nodes.find((n) => n.id === nodeId);
 }