fix: address Copilot review round 36 — re-derive the branch when the union changes underneath

`branchIndex` is a position, and a tool refreshed in place keeps its
`resetKey` — so a union reordered or rewritten with the same number of
branches left the index pointing at a different shape than the one the values
describe: the picker showed SMS while Execute submitted the email arguments.
The selection is now re-derived from the values whenever the alternatives
themselves change, keyed by their labels and declared fields.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx

@@ -2355,6 +2355,42 @@ describe("SchemaForm multiline strings (#2042)", () => {
       );
     });
 
+    it("re-derives the selection when the branches are reordered in place", () => {
+      // Same `resetKey`, same branch count — only the order changed, so a
+      // numeric index would now point at the other shape and the picker would
+      // show SMS while `values` describe email.
+      const reversed: InspectorFormSchema = {
+        ...UNION_SCHEMA,
+        anyOf: [...(UNION_SCHEMA.anyOf ?? [])].reverse(),
+      };
+      const values = { note: "hi", kind: "email", address: "a@b.c" };
+      const { rerender } = renderWithMantine(
+        <SchemaForm
+          schema={UNION_SCHEMA}
+          values={values}
+          onChange={vi.fn()}
+          resetKey="same-tool"
+        />,
+      );
+      expect(screen.getByRole("textbox", { name: /Address/ })).toBeTruthy();
+
+      rerender(
+        <SchemaForm
+          schema={reversed}
+          values={values}
+          onChange={vi.fn()}
+          resetKey="same-tool"
+        />,
+      );
+      // Still the email branch — the one the values describe — not whatever
+      // now sits at the old index.
+      expect(
+        (screen.getByRole("textbox", { name: /Variant/ }) as HTMLInputElement)
+          .value,
+      ).toBe("email");
+      expect(screen.getByRole("textbox", { name: /Address/ })).toBeTruthy();
+    });
+
     it("renders no picker for a single-branch union but still shows its fields", () => {
       const schema: InspectorFormSchema = {
         type: "object",

clients/web/src/components/groups/SchemaForm/SchemaForm.tsx

@@ -623,6 +623,18 @@ export function SchemaForm({
   const [branchIndex, setBranchIndex] = useState(
     () => selectBranchIndex(branches, values) ?? 0,
   );
+  // The alternatives themselves, as a stable key. A tool refreshed in place
+  // keeps its `resetKey`, so nothing else notices that the union underneath was
+  // reordered or rewritten — and a numeric index then points at a different
+  // branch than the one whose values are held, showing SMS while submitting
+  // email. Re-derived from the values, which is where the answer actually is.
+  const branchesKey = branches
+    .map((branch) => `${branch.label}:${branch.declaredFields.join(",")}`)
+    .join("|");
+  useValueChange(branchesKey, () =>
+    setBranchIndex(selectBranchIndex(branches, values) ?? 0),
+  );
+
   // A form reused for another entity can be handed a shorter union, so the
   // index is clamped rather than trusted — `resetKey` resets it below, but a
   // caller that omits it (the elicitation panels mount fresh) supplies none.