Batch operations #48

Context

core.add and core.remove work one entity at a time, and each call emits its own entities:changed event. Loading 1,000 entities emits 1,000 events, and each one triggers a full repaint in the SVG renderer (see #47). Deleting a multi-selection has the same problem.

core.updateMany already exists and emits a single { type: 'update', ids } event. This issue adds the same for add and remove, and a generic batch for mixed changes.

What changes

New methods on SpatialCore:

core.addMany(entities: Entity[]): void;      // one { type: 'add', ids } event
core.removeMany(ids: EntityId[]): void;      // one { type: 'remove', ids } event
                                             // + one selection:change if any removed id was selected

Optional: a generic batch. It collects every change made inside the callback and emits at most one event per type at the end:

core.batch(() => {
  core.add(a);
  core.update(b.id, { bounds });
  core.remove(c.id);
});
// emits: add [a], update [b], remove [c] — once each, after the callback

Proposal: ship addMany and removeMany first (small and clear), and discuss batch in the comments. Nested batches and errors thrown inside the callback need a decision.

Affected files

Invariants to preserve

• Event payload shapes do not change: { type, ids } and { selection }.
• Empty input emits no event, the same as updateMany today.
• removeMany removes ids from the selection, the same as remove.
• Insertion order is preserved by addMany (it defines z-order).
• No production dependencies; core stays framework-agnostic.

Implementation checklist

• addMany with tests (one event, ids in order, empty input)
• removeMany with tests (one event, selection updated with one selection:change, unknown ids ignored)
• Docs in the core README
• Changeset (minor for core)
• Run pnpm typecheck && pnpm test && pnpm lint
• Optional: discuss and implement batch (may be a follow-up issue)