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/build-embedded.sh

@@ -0,0 +1,56 @@
+#!/bin/sh
+#
+# Cross-compiles the PlayDate target for Playdate hardware (Embedded Swift,
+# ARM Cortex-M7) as a compile-only check that the bindings stay within the
+# Embedded Swift subset. Linking is left to game projects.
+#
+# Requirements:
+# - A Swift toolchain with the embedded stdlib for armv7em-none-none-eabi
+#   (a swift.org development snapshot; Xcode's toolchain does not ship it).
+#   Override the binary with SWIFT_BIN, otherwise `swift` from PATH is used.
+# - Arm GNU toolchain (arm-none-eabi) C headers, because pd_api.h includes
+#   libc headers that bare-metal builds resolve against newlib. Override the
+#   directory with ARM_NONE_EABI_INCLUDE, otherwise common install locations
+#   are searched.
+# - The "playdate" pkg-config module (Scripts/install-pkgconfig.sh).
+
+set -eu
+
+swift_bin="${SWIFT_BIN:-swift}"
+
+include_dir="${ARM_NONE_EABI_INCLUDE:-}"
+if [ -z "$include_dir" ]; then
+    for candidate in \
+        /usr/local/playdate/gcc-arm-none-eabi-*/arm-none-eabi/include \
+        /Applications/ArmGNUToolchain/*/arm-none-eabi/arm-none-eabi/include \
+        /usr/lib/arm-none-eabi/include \
+        /usr/include/newlib; do
+        if [ -f "$candidate/stdlib.h" ]; then
+            include_dir="$candidate"
+            break
+        fi
+    done
+fi
+
+if [ -z "$include_dir" ] || [ ! -f "$include_dir/stdlib.h" ]; then
+    echo "error: arm-none-eabi C headers not found." >&2
+    echo "Install the Arm GNU toolchain or set ARM_NONE_EABI_INCLUDE." >&2
+    exit 1
+fi
+
+echo "Using swift: $swift_bin ($($swift_bin --version 2>/dev/null | head -1))"
+echo "Using arm-none-eabi headers: $include_dir"
+
+# --build-system native stops after compilation; the default build system
+# also tries to merge objects with the host linker, which cannot process
+# bare-metal ELF objects.
+exec "$swift_bin" build \
+    --build-system native \
+    --target PlayDate \
+    --triple armv7em-none-none-eabi \
+    -Xswiftc -enable-experimental-feature -Xswiftc Embedded \
+    -Xswiftc -wmo \
+    -Xcc -I"$include_dir" \
+    -Xcc -mcpu=cortex-m7 \
+    -Xcc -mfloat-abi=hard \
+    -Xcc -mfpu=fpv5-sp-d16