Fix macOS CI: move to Xcode 26.4 on the macos-26 runner

The macos-latest image rotated out Xcode 16.4, so `xcode-select --switch
/Applications/Xcode_16.4.app` fails before the build even starts. Move
the macOS job to Xcode 26.4 on macos-26, matching JsRuntimeHost and
BabylonNative (which already run this pairing green).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Copilot

.github/workflows/ci.yml

@@ -14,11 +14,11 @@ jobs:
       platform: x64
 
   # ── macOS ─────────────────────────────────────────────────────
-  macOS_Xcode164:
+  macOS_Xcode264:
     uses: ./.github/workflows/build-macos.yml
     with:
-      xcode-version: '16.4'
-      runs-on: macos-latest
+      xcode-version: '26.4'
+      runs-on: macos-26
 
   # ── Linux ─────────────────────────────────────────────────────
   Ubuntu_gcc: