test(derived): enter edit mode, and add the positive control

The screen opens read-only until the edit button is pressed, so the case
asserted nothing about the column: no cell opened an editor. It now enters edit
mode first, asserts the editor's absence directly rather than typing at
whatever happens to be focused, and a sibling case proves a physical column in
the same grid does open one - so the silence is about the column, not the
screen.

Mutating the mock to drop the READONLY rule fails the derived case and passes
the physical one; restoring it passes both (5/5, ~48s).

client/cypress/e2e/derived-variables.cy.ts

@@ -78,14 +78,25 @@ context('derived variables: ', function () {
 
   it('4 | a derived column does not accept an edit', () => {
     openEditor('TESTDATA.DERIVED_DEMO')
+    enterEditMode()
 
-    getCellByHeaderAndRow(0, 'REGION')
-      .dblclick({ force: true })
-      .then(() => {
-        // a read-only cell opens no editor, so nothing is focused to type into
-        cy.focused().type('North{enter}')
-        getCellByHeaderAndRow(0, 'REGION').should('have.text', 'West')
-      })
+    getCellByHeaderAndRow(0, 'REGION').dblclick({ force: true })
+    // a read-only cell opens no editor at all, so there is nothing to type
+    // into and the value cannot move
+    cy.get('.handsontableInput').should('not.exist')
+    getCellByHeaderAndRow(0, 'REGION').should('have.text', 'West')
+  })
+
+  /**
+   * The positive control for the case above: same screen, same edit mode, a
+   * physical column - so its silence is about the column, not the screen.
+   */
+  it('5 | a physical column in the same grid does accept an edit', () => {
+    openEditor('TESTDATA.DERIVED_DEMO')
+    enterEditMode()
+
+    getCellByHeaderAndRow(0, 'STATE').dblclick({ force: true })
+    cy.get('.handsontableInput').should('exist')
   })
 })
 
@@ -115,6 +126,13 @@ const openEditor = (libMem: string) => {
   cy.wait(2500)
 }
 
+/** The EDIT screen opens read-only; this is the button that starts an edit. */
+const enterEditMode = () => {
+  cy.get('.btnCtrl button.btn-primary', { timeout: longerCommandTimeout })
+    .click()
+    .then(() => cy.wait(2500))
+}
+
 const openFilterPopup = () => {
   cy.contains('button.btnView', 'Filter', {
     timeout: longerCommandTimeout