Build the macOS shim from source in CI

Until now a change to native/swift did not reach the macOS job at all. That
job loaded the committed dylib, so the pixel baselines described whatever
was last rebuilt, and a Swift change could not turn it red however wrong it
was -- it only went red on the ABI version check, and only until the
binaries PR was merged. The Linux job has built from source all along and
has not had that gap.

Only the runner architecture is built, since this is the only place it is
loaded. What ships is still loaded and still checked by the native job,
which is now the only thing testing the committed binaries and says so.

Author: SimonCropp

.github/workflows/test.yml

@@ -109,6 +109,25 @@ jobs:
           mkdir -p src/DiffEngineViewer.Linux/runtimes/linux-x64/native
           cp native/build/linux-x64/libdiffengine_viewer.so src/DiffEngineViewer.Linux/runtimes/linux-x64/native/
 
+      # Same reason as the Linux build above, and the same trade. Before it, a change to
+      # native/swift did not reach this job at all until build-native had run and its binaries PR
+      # had been merged, so the macOS baselines described the previous rebuild and a Swift change
+      # could not go red here however wrong it was.
+      #
+      # Only this runner's own architecture, because this is the only thing that loads it. The
+      # committed universal binary is still loaded and still checked, by the native job below —
+      # which is the job that exists to prove what actually ships.
+      #
+      # Not cached: this is six Swift files against system frameworks, twenty-odd seconds, and a
+      # stale cache here would reintroduce exactly the staleness the step removes.
+      - name: Build native renderer
+        if: runner.os == 'macOS'
+        run: |
+          swift build -c release --package-path native/swift
+          mkdir -p src/DiffEngineViewer.Mac/runtimes/osx-arm64/native
+          cp "$(swift build -c release --package-path native/swift --show-bin-path)/libdiffengine_viewer.dylib" \
+            src/DiffEngineViewer.Mac/runtimes/osx-arm64/native/
+
       # Release-NotWindows drops the WinForms tray and its tests from the solution.
       - name: Build
         run: dotnet build src --configuration Release-NotWindows
@@ -152,10 +171,11 @@ jobs:
           if-no-files-found: ignore
           retention-days: 14
 
-  # The jobs above only ever load the x64 natives, and the Linux one rebuilds its own from source.
-  # This job exists so the arm64 binaries are actually loaded somewhere, which is what catches a
-  # wrong architecture, a file corrupted by a text mode checkout, or an unsatisfied runtime
-  # dependency.
+  # Neither job above loads a committed binary any more: both build the shim from source so their
+  # pixel baselines track it. That leaves nothing testing what actually ships, which is this job.
+  # It loads the committed binaries as they are, which is what catches a wrong architecture, a file
+  # corrupted by a text mode checkout, an unsatisfied runtime dependency, or simply a rebuild that
+  # was never merged.
   #
   # It runs DiffEngineViewer.Tests rather than the whole suite: that is where the native smoke test
   # lives, it takes seconds, and it gives the screen and IPC tests some cross architecture coverage