test(editor): assert the derived block marker, not a column name

derived-variables.cy.ts:97 expected dc-derived-first on the REGION header,
because the comment assumed REGION opens the computed block. The grid follows
MPE_DERIVED order, and that order is LINE_VALUE, REGION, REGION_LABEL - so the
marker was on LINE_VALUE and correct. The spec asserted the wrong column, and
the development pipeline has been red on it since.

Dumping the header row on a local mock estate (sasjs/server desktop mode,
RUN_TIMES=js) shows:

  6 | LINE_VALUE | dc-derived-header dc-derived-first
  7 | REGION | dc-derived-header
  8 | REGION_LABEL | dc-derived-header beforeHiddenColumn htLastVisibleHeader

and the same dump against the unmodified release/7.19 client, so this was never
a client defect.

The assertion now states the invariant instead: at least one derived header, the
first of them opens the block, and no other one carries the marker. That holds
whatever order the control table returns the variables in.

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

@@ -93,8 +93,29 @@ context('derived variables: ', function () {
       'not.have.class',
       'dc-derived-cell'
     )
-    // REGION is the first derived column, so it opens the computed block
-    headerCell('REGION').should('have.class', 'dc-derived-first')
+    // The computed block opens at whichever derived column comes first in the
+    // grid - MPE_DERIVED order, so LINE_VALUE here, not REGION. Assert the
+    // invariant rather than a name: exactly one derived header carries the
+    // marker, and it is the first one.
+    cy.get('.ht_clone_top .htCore thead tr th').then(($ths) => {
+      const derived = [...$ths].filter((th) =>
+        th.classList.contains('dc-derived-header')
+      )
+      expect(derived.length, 'derived headers').to.be.greaterThan(0)
+      expect(
+        derived[0].classList.contains('dc-derived-first'),
+        'the first derived header opens the block'
+      ).to.be.true
+      derived
+        .slice(1)
+        .forEach(
+          (th) =>
+            expect(
+              th.classList.contains('dc-derived-first'),
+              'only the first derived header carries the marker'
+            ).to.be.false
+        )
+    })
 
     // the hover text carries the label, which is what says what the column is:
     // the configured VAR_LABEL where there is one, otherwise the generated