Add CI: host tests, consumer smoke test, Embedded Swift check

Three jobs in GitHub Actions:
- Build & test on macOS against SDK headers fetched from Panic (cached),
  with the pkg-config module generated into PKG_CONFIG_PATH.
- A consumer smoke test (Scripts/consumer-test.sh) generating a scratch
  package that depends on play-date, guarding setting propagation.
- An Embedded Swift cross-compile for armv7em-none-none-eabi
  (Scripts/build-embedded.sh) on a swift.org snapshot toolchain — the
  check that enforces the Embedded Swift subset promise. Verified locally:
  the full module compiles for the device triple with no violations.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Scripts/consumer-test.sh

@@ -0,0 +1,48 @@
+#!/bin/sh
+#
+# Verifies the package works as a SwiftPM dependency: generates a scratch
+# executable package that depends on play-date, imports both modules, and
+# runs it. This guards the pkg-config setup — target settings that don't
+# propagate to consumers (or manifest validation failures) surface here,
+# not on this package's own build.
+
+set -eu
+
+package_dir="$(cd "$(dirname "$0")/.." && pwd)"
+scratch_dir="$(mktemp -d)"
+trap 'rm -rf "$scratch_dir"' EXIT
+
+mkdir -p "$scratch_dir/Sources/consumer"
+
+cat > "$scratch_dir/Package.swift" <<EOF
+// swift-tools-version: 6.4
+import PackageDescription
+
+let package = Package(
+    name: "consumer",
+    dependencies: [
+        .package(path: "$package_dir"),
+    ],
+    targets: [
+        .executableTarget(
+            name: "consumer",
+            dependencies: [.product(name: "PlayDate", package: "play-date")]
+        ),
+    ]
+)
+EOF
+
+cat > "$scratch_dir/Sources/consumer/main.swift" <<'EOF'
+import CPlaydate
+import PlayDate
+
+// Touch a type from each module to prove both import and link.
+let event = Playdate.SystemEvent(event: kEventInit, argument: 0)
+let buttons: Playdate.System.Buttons = [.a, .up]
+print(event != nil && buttons.contains(.a) ? "ok" : "broken")
+EOF
+
+cd "$scratch_dir"
+output="$(swift run 2>&1 | tail -1)"
+echo "consumer output: $output"
+[ "$output" = "ok" ]