Add Swift Source Compatibility CI workflow

Implements GitHub Actions workflow to test MistKit against multiple Swift toolchain versions (6.0, 6.1, 6.2, and 6.3-nightly) for source compatibility validation.

- Tests Swift 6.0, 6.1, 6.2 (stable) and 6.3-nightly
- Build-only approach using official Swift Docker containers
- Nightly failures allowed via continue-on-error flag
- Follows SyndiKit source compatibility suite pattern
- Runs on push to main, pull requests, and manual dispatch
- Uses explicit Swift 6 language mode for all builds

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: leogdion

.github/workflows/swift-source-compat.yml

@@ -0,0 +1,32 @@
+name: Swift Source Compatibility
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  swift-source-compat-suite:
+    name: Test Swift ${{ matrix.container }} For Source Compatibility Suite
+    runs-on: ubuntu-latest
+    if: ${{ !contains(github.event.head_commit.message, 'ci skip') }}
+    continue-on-error: ${{ contains(matrix.container, 'nightly') }}
+
+    strategy:
+      fail-fast: false
+      matrix:
+        container:
+          - swift:6.0
+          - swift:6.1
+          - swift:6.2
+          - swiftlang/swift:nightly-6.3-noble
+
+    container: ${{ matrix.container }}
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Test Swift 6.x For Source Compatibility
+        run: swift build --disable-sandbox --verbose --configuration release -Xswiftc -swift-version -Xswiftc 6