before functional changes by codex

test/edge/pfr-measurement-grid.edge.test.js

@@ -0,0 +1,15 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+
+const { Reactor_PFR } = require('../../src/specificClass');
+const { makeReactorConfig } = require('../helpers/factories');
+
+test('oxygen measurement at exact reactor length overflows PFR grid index (known bounds gap)', () => {
+  const reactor = new Reactor_PFR(
+    makeReactorConfig({ reactor_type: 'PFR', length: 10, resolution_L: 5, n_inlets: 1 }),
+  );
+
+  assert.throws(() => {
+    reactor._updateMeasurement('quantity (oxygen)', 2.5, 10, {});
+  });
+});