Add GitHub Actions workflow to build and run demos on macOS

- Swift demo: build + run all 5 demos via SPM on macOS
- ObjC demo: compile with clang + run all 5 demos on macOS
- Swift tests: run full test suite on macOS (Network.framework available)
- Triggers: push/PR to main, manual workflow_dispatch
- Minimal permissions (contents: read)

Co-authored-by: dustturtle <2305214+dustturtle@users.noreply.github.com>

Author: Copilot

.github/workflows/demo.yml

@@ -0,0 +1,55 @@
+name: Demo
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  swift-demo:
+    name: Swift Demo (macOS)
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build Swift Demo
+        run: swift build --target SwiftDemo
+
+      - name: Run Swift Demo (all demos)
+        run: |
+          printf 'a\n\n\n\n\n\nq\n' | swift run SwiftDemo
+
+  objc-demo:
+    name: ObjC Demo (macOS)
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build ObjC Demo
+        run: |
+          clang -framework Foundation -framework Network \
+                -I ObjC/NWAsyncSocketObjC/include \
+                ObjC/NWAsyncSocketObjC/NWStreamBuffer.m \
+                ObjC/NWAsyncSocketObjC/NWSSEParser.m \
+                ObjC/NWAsyncSocketObjC/NWReadRequest.m \
+                ObjC/NWAsyncSocketObjC/GCDAsyncSocket.m \
+                ObjC/ObjCDemo/main.m \
+                -o ObjCDemo
+
+      - name: Run ObjC Demo (all demos)
+        run: |
+          printf 'a\n\n\n\n\n\nq\n' | ./ObjCDemo
+
+  swift-tests:
+    name: Swift Tests (macOS)
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Run Swift tests
+        run: swift test