Run tests in CI on Linux

Adds a Gitea Actions workflow running swift test in a swift:6.2 container.

The kit did not build without Apple frameworks, so the two files that need
them are wrapped in #if canImport, and the SwiftUI app target is added to
Package.swift only on macOS. Neither can exist in a Linux container.

This costs no coverage: all 43 tests are about file format and none touch
the Contacts or EventKit readers. The integration those readers represent
remains verifiable only on a Mac.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01C5X1tYo9oxAfvoiFMh1QQr

.gitea/workflows/test.yml

@@ -0,0 +1,28 @@
+name: Tests
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    container:
+      # Pinned rather than tracking latest: a toolchain bump should be a commit,
+      # not a surprise on an unrelated push.
+      image: swift:6.2
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Toolchain
+        run: swift --version
+
+      # This only covers IPodSyncKit — the format logic, which is where every
+      # test lives. The Contacts and EventKit readers are compiled out on Linux
+      # and can only be exercised on a Mac.
+      - name: Build
+        run: swift build --build-tests
+
+      - name: Test
+        run: swift test