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/commands/annotationCommands.ts

@@ -0,0 +1,184 @@
+/**
+ * Groups and comments: boxes drawn behind a graph. Both kinds take the same commands, because they
+ * hold the same thing – which list they live in is the whole difference, and it is a parameter.
+ */
+import {
+  type AnnotationKind,
+  annotationList,
+  annotationsOf,
+  type GraphAnnotation,
+  MIN_ANNOTATION_SIZE,
+} from '../graph/Graph';
+import { newId } from '../ids';
+import type { Project } from '../model/Project';
+import { type Removed, removeWhere, restoreAt } from '../util/list';
+import { type Command, CommandError } from './Command';
+import { requireGraph } from './require';
+
+/** What each kind is called where a user reads it – a "note" is a word only the model uses. */
+export function annotationNoun(kind: AnnotationKind): string {
+  return kind === 'group' ? 'group' : 'comment';
+}
+
+export interface AnnotationBox {
+  x: number;
+  y: number;
+  w: number;
+  h: number;
+}
+
+/** Rounded to whole units and never smaller than the minimum, wherever a box comes from. */
+export function normalizeBox(box: AnnotationBox): AnnotationBox {
+  return {
+    x: Math.round(box.x),
+    y: Math.round(box.y),
+    w: Math.max(Math.round(box.w), MIN_ANNOTATION_SIZE.w),
+    h: Math.max(Math.round(box.h), MIN_ANNOTATION_SIZE.h),
+  };
+}
+
+function requireAnnotation(
+  project: Project,
+  assetId: string,
+  kind: AnnotationKind,
+  id: string,
+): GraphAnnotation {
+  const found = annotationsOf(requireGraph(project, assetId), kind).find((a) => a.id === id);
+  if (!found) throw new CommandError(`${annotationNoun(kind)} ${id} not found`);
+  return found;
+}
+
+export class AddAnnotationCommand implements Command {
+  readonly label: string;
+  readonly annotation: GraphAnnotation;
+
+  constructor(
+    private readonly assetId: string,
+    private readonly kind: AnnotationKind,
+    box: AnnotationBox,
+    text: string,
+  ) {
+    this.annotation = { id: newId(), ...normalizeBox(box), text };
+    this.label = `Add ${annotationNoun(kind)}`;
+  }
+
+  execute(project: Project): void {
+    annotationList(requireGraph(project, this.assetId), this.kind).push(this.annotation);
+  }
+
+  undo(project: Project): void {
+    const list = annotationList(requireGraph(project, this.assetId), this.kind);
+    removeWhere(list, (a) => a.id === this.annotation.id);
+  }
+}
+
+/**
+ * Takes the box away and nothing else: the nodes it framed stay where they are. A box that
+ * silently deleted ten nodes with it would be the most expensive surprise in the editor – whoever
+ * wants the nodes gone selects them and presses Delete, which the graph could already do.
+ */
+export class RemoveAnnotationCommand implements Command {
+  readonly label: string;
+  private removed: Removed<GraphAnnotation>[] = [];
+
+  constructor(
+    private readonly assetId: string,
+    private readonly kind: AnnotationKind,
+    private readonly id: string,
+  ) {
+    this.label = `Delete ${annotationNoun(kind)}`;
+  }
+
+  execute(project: Project): void {
+    const list = annotationList(requireGraph(project, this.assetId), this.kind);
+    this.removed = removeWhere(list, (a) => a.id === this.id);
+    if (this.removed.length === 0)
+      throw new CommandError(`${annotationNoun(this.kind)} ${this.id} not found`);
+  }
+
+  undo(project: Project): void {
+    restoreAt(annotationList(requireGraph(project, this.assetId), this.kind), this.removed);
+  }
+}
+
+/**
+ * Move and resize in one command, since both are the same edit to the same four numbers. As with
+ * `MoveNodeCommand` the previous box is read at execution time, so a caller that previewed the
+ * drag must put the box back before executing.
+ */
+export class SetAnnotationBoxCommand implements Command {
+  readonly label: string;
+  private previous: AnnotationBox = { x: 0, y: 0, w: 0, h: 0 };
+
+  constructor(
+    private readonly assetId: string,
+    private readonly kind: AnnotationKind,
+    private readonly id: string,
+    private readonly box: AnnotationBox,
+    label?: string,
+  ) {
+    this.label = label ?? `Move ${annotationNoun(kind)}`;
+  }
+
+  execute(project: Project): void {
+    const a = requireAnnotation(project, this.assetId, this.kind, this.id);
+    this.previous = { x: a.x, y: a.y, w: a.w, h: a.h };
+    Object.assign(a, normalizeBox(this.box));
+  }
+
+  undo(project: Project): void {
+    Object.assign(requireAnnotation(project, this.assetId, this.kind, this.id), this.previous);
+  }
+}
+
+export class SetAnnotationTextCommand implements Command {
+  readonly label: string;
+  private previous = '';
+
+  constructor(
+    private readonly assetId: string,
+    private readonly kind: AnnotationKind,
+    private readonly id: string,
+    private readonly text: string,
+  ) {
+    this.label = kind === 'group' ? 'Set title' : 'Set comment';
+  }
+
+  execute(project: Project): void {
+    const a = requireAnnotation(project, this.assetId, this.kind, this.id);
+    this.previous = a.text;
+    a.text = this.text;
+  }
+
+  undo(project: Project): void {
+    requireAnnotation(project, this.assetId, this.kind, this.id).text = this.previous;
+  }
+}
+
+/** `null` clears the colour, which is how a box goes back to the theme's default. */
+export class SetAnnotationColorCommand implements Command {
+  readonly label = 'Set color';
+  private previous: number | null = null;
+
+  constructor(
+    private readonly assetId: string,
+    private readonly kind: AnnotationKind,
+    private readonly id: string,
+    private readonly color: number | null,
+  ) {}
+
+  execute(project: Project): void {
+    const a = requireAnnotation(project, this.assetId, this.kind, this.id);
+    this.previous = a.color ?? null;
+    apply(a, this.color);
+  }
+
+  undo(project: Project): void {
+    apply(requireAnnotation(project, this.assetId, this.kind, this.id), this.previous);
+  }
+}
+
+function apply(annotation: GraphAnnotation, color: number | null): void {
+  if (color === null) delete annotation.color;
+  else annotation.color = color;
+}