fix(subsUnion): force-bundle turf deps used only by the worker thread

Meteor's build only includes npm packages it sees imported/required
somewhere in its own traced module graph. private/workers/unionWorker.js
is a plain asset (loaded at runtime via new Worker(), never
import/require'd from Meteor-compiled code), so @turf/circle,
@turf/union and @turf/truncate got silently dropped from
programs/server/node_modules — the worker crashed on every call with
"Cannot find module '@turf/circle'", which incrementalAdd/process
swallowed and stored as a "null" union (typeof null === 'object', so
the null-check that guards against failed unions doesn't catch it).
Side-effect imports here force Meteor to bundle them.

imports/startup/server/calcUnionAsync.js

@@ -1,6 +1,16 @@
 import path from 'path';
 import { Worker } from 'worker_threads';
 
+// Meteor only bundles the npm packages it sees imported/required somewhere in
+// its own compiled module graph. The actual usage of these three is inside
+// private/workers/unionWorker.js, a plain asset file Meteor's bundler never
+// traces (it's loaded at runtime via new Worker(), not import/require here).
+// Without this, `meteor build` drops them from programs/server/node_modules
+// and the worker fails at startup with "Cannot find module '@turf/circle'".
+import '@turf/circle';
+import '@turf/union';
+import '@turf/truncate';
+
 const WORKER_PATH = path.resolve(process.cwd(), 'assets/app/workers/unionWorker.js');
 
 /**