ci: build against device SDK — CoreSimulator hangs on external-volume Xcode

Simulator-destination builds flap on CoreSimulatorService for an hour then
die. Device SDK with signing disabled gives identical compile coverage.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -36,33 +36,24 @@ jobs:
       - name: Generate Xcode project
         run: xcodegen generate
 
-      - name: Build (${{ matrix.configuration }}, iOS Simulator)
+      # Device SDK, signing disabled: CoreSimulator on the runner Mac is unreliable
+      # (Xcode on an external volume can't discover simulator runtimes and hangs).
+      # Compilation coverage is identical; no tests run on simulator yet anyway.
+      - name: Build (${{ matrix.configuration }}, iOS device SDK)
         run: |
           set -o pipefail
           xcodebuild \
             -project Jarvis.xcodeproj \
             -scheme Jarvis \
             -configuration ${{ matrix.configuration }} \
-            -sdk iphonesimulator \
-            -destination 'generic/platform=iOS Simulator' \
+            -sdk iphoneos \
+            -destination 'generic/platform=iOS' \
             -derivedDataPath build \
+            COMPILATION_CACHE_ENABLE_CACHING=NO \
             CODE_SIGNING_ALLOWED=NO \
             CODE_SIGNING_REQUIRED=NO \
             build
 
-      # Compiles the test bundle. No test targets exist yet (see docs/BACKLOG.md #13).
-      # Replace with `xcodebuild test` once a JarvisTests target is added to project.yml.
-      - name: Build for Testing (Debug)
-        if: matrix.configuration == 'Debug'
-        run: |
-          set -o pipefail
-          xcodebuild \
-            -project Jarvis.xcodeproj \
-            -scheme Jarvis \
-            -configuration Debug \
-            -sdk iphonesimulator \
-            -destination 'platform=iOS Simulator,OS=latest,name=iPhone 16' \
-            -derivedDataPath build \
-            CODE_SIGNING_ALLOWED=NO \
-            CODE_SIGNING_REQUIRED=NO \
-            build-for-testing
+      # No test targets exist yet (see docs/BACKLOG.md #13). When a JarvisTests
+      # target is added, switch this to `xcodebuild test` on a simulator — after
+      # fixing CoreSimulator on the runner (Xcode must live on the boot volume).