ci: build on macos-26 (Xcode 26) so the Python scheme's module map resolves

main's CI has been red since the dual-backend merge: the Build (Python
backend) step fails precompiling ColumbaApp's bridging header with
'module Python … clang importer creation failed' (exit 65). The embedded-
Python (Columba) scheme's Python.framework clang module map only resolves
under Xcode 26.x; the runner was pinned to macos-15/Xcode 16. The fix lived
on the combined #82 branch, which was closed (not merged) — so it was
stranded, like the interop-harness fixes.

- runs-on: macos-15 -> macos-26 (Xcode 26.x).
- Select Xcode: pick newest (Xcode_*) instead of Xcode_16* so 26.x wins.
- Add an explicit 'Resolve Swift packages' step to avoid the fresh-runner
  package-resolution race for the test target.

Recovered verbatim from origin/feat/dual-backend-arch (#82). Fixes CI for
main and unblocks the other open PRs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: torlando-agent[bot]

.github/workflows/tests.yml

@@ -12,7 +12,11 @@ concurrency:
 
 jobs:
   test:
-    runs-on: macos-15
+    # macos-26 carries Xcode 26.x, which the embedded-Python (Columba) scheme
+    # needs — the Python.framework clang module map fails to resolve under
+    # Xcode 16.x ("module 'Python' … not defined in any loaded module map"),
+    # which is what devs build with locally (26.x). See the Select Xcode step.
+    runs-on: macos-26
     steps:
       - uses: actions/checkout@v4
 
@@ -32,11 +36,26 @@ jobs:
 
       - name: Select Xcode
         run: |
-          XCODE=$(ls -d /Applications/Xcode_16*.app 2>/dev/null | sort -V | tail -1)
+          echo "Available Xcodes:"; ls -d /Applications/Xcode_*.app 2>/dev/null || true
+          # Pick the newest Xcode on the runner (was pinned to 16.x, which can't
+          # build the Python scheme — see runs-on comment). Version-sort so 26.x
+          # wins over 16.x regardless of exact point release.
+          XCODE=$(ls -d /Applications/Xcode_*.app 2>/dev/null | sort -V | tail -1)
           echo "Using $XCODE"
           sudo xcode-select -s "$XCODE"
           xcodebuild -version
 
+      # Resolve the local Swift packages (reticulum-swift / LXMF-swift / LXST-swift)
+      # once, up front. Without this the `xcodebuild test` action can intermittently
+      # fail to resolve LXMFSwift/ReticulumSwift for the ColumbaAppTests target even
+      # though the app build steps resolved them fine — a fresh-runner resolution
+      # race. One explicit resolve makes every later build/test step reuse it.
+      - name: Resolve Swift packages
+        run: |
+          xcodebuild -resolvePackageDependencies \
+            -project Columba.xcodeproj \
+            -scheme Columba
+
       - name: Find simulator
         id: sim
         run: |