Document SwiftPM's #include <json.hpp> form and add CI coverage

Package.swift declares the json target with path: "single_include/nlohmann",
so SwiftPM consumers must write #include <json.hpp>; the <nlohmann/json.hpp>
form used throughout the rest of this documentation does not resolve. The
SwiftPM section of package_managers.md only had the summary box and did not
mention this, nor the #4650 linker problem: because the json target ships
only headers, SwiftPM/Xcode look for a json.o that is never built, so a
consumer needs at least one .cpp file of its own.

Add a usage example (docs/mkdocs/docs/integration/swift/) that depends on
the json product, includes <json.hpp>, and works around the linker problem
with its own example.cpp; verified it builds with `swift build` against a
local checkout. Also raise the deployment target from the deprecated
.watchOS(.v4) to .watchOS(.v9): Swift 6.4 warns "'v4' is deprecated: watchOS
9.0 is the oldest supported version" on every `swift package` invocation,
and `swift package dump-package` is now clean. This is the one behavior
change in this commit: it raises the minimum watchOS version the package
declares.

Add a macOS CI job that runs `swift package dump-package` (so a future
deprecation warning fails CI) and builds the new documentation example
against the checked-out repository with `swift build`, so a change to
Package.swift or the example is caught before release. No workflow
previously referenced Swift or Package.swift.

Adding an in-repository placeholder .cpp to the library itself, so the
issue does not occur for consumers at all, was proposed and declined in
#4650; the documented workaround is a consumer-side .cpp file instead.

#5716 item 3

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/macos.yml

@@ -71,3 +71,38 @@ jobs:
         run: cmake --build build --parallel 10
       - name: Test
         run: cd build ; ctest -j 10 --output-on-failure
+
+  swiftpm:
+    runs-on: macos-15
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+      - name: Check that Package.swift resolves without a deprecation warning
+        run: swift package dump-package
+      - name: Build the SwiftPM documentation example against this checkout
+        run: |
+          mkdir -p /tmp/json-swiftpm-consumer/Sources/MyLibrary
+          cp docs/mkdocs/docs/integration/swift/example.cpp /tmp/json-swiftpm-consumer/Sources/MyLibrary/example.cpp
+          cat > /tmp/json-swiftpm-consumer/Package.swift << EOF
+          // swift-tools-version: 5.9
+          import PackageDescription
+
+          let package = Package(
+              name: "MyPackage",
+              dependencies: [
+                  .package(path: "${{ github.workspace }}")
+              ],
+              targets: [
+                  .target(
+                      name: "MyLibrary",
+                      dependencies: [
+                          .product(name: "json", package: "json")
+                      ],
+                      publicHeadersPath: "."
+                  )
+              ]
+          )
+          EOF
+          cd /tmp/json-swiftpm-consumer
+          swift build