add build instructions and helper script

scripts/build-core.sh

@@ -0,0 +1,64 @@
+#!/usr/bin/env sh
+# Builds the Haskell core library for one target and puts it where the app
+# build expects it. Run from the repo root.
+#
+#   ./scripts/build-core.sh android-aarch64
+#   ./scripts/build-core.sh android-armv7a
+#   ./scripts/build-core.sh desktop-linux [sqlite|postgres]
+#   ./scripts/build-core.sh desktop-mac [x86_64|arm64] [sqlite|postgres]
+#   ./scripts/build-core.sh desktop-windows [x86_64]
+#   ./scripts/build-core.sh ios                                  # macOS only
+#
+# nix targets (android-*, ios) need Nix >= 2.22 with flakes enabled and
+# https://cache.iog.io as a substituter, otherwise the cross-GHC is compiled
+# from source (12-24 hours). Desktop targets need ghcup's GHC 9.6.3 + cabal
+# and a cabal.project.local, see docs/CONTRIBUTING.md.
+set -eu
+
+android_libs="apps/multiplatform/common/src/commonMain/cpp/android/libs"
+
+# $1 = flake attribute path after .#hydraJobs., $2 = zip in ./result, $3 = destination
+nix_unzip() {
+  nix build ".#hydraJobs.$1"
+  mkdir -p "$3"
+  unzip -o "result/$2" -d "$3"
+}
+
+build_android() {
+  arch="$1"
+  abi="$2"
+  nix_unzip "x86_64-linux.\"${arch}-android:lib:simplex-chat\"" \
+            "pkg-${arch}-android-libsimplex.zip" "$android_libs/$abi"
+  nix_unzip "x86_64-linux.\"${arch}-android:lib:support\"" \
+            "pkg-${arch}-android-libsupport.zip" "$android_libs/$abi"
+}
+
+target="${1:-}"
+if [ $# -gt 0 ]; then shift; fi
+
+case "$target" in
+  android-aarch64|arm64-v8a)
+    build_android aarch64 arm64-v8a
+    ;;
+  android-armv7a|armeabi-v7a)
+    build_android armv7a armeabi-v7a
+    ;;
+  desktop-linux)
+    scripts/desktop/build-lib-linux.sh "$@"
+    ;;
+  desktop-mac)
+    scripts/desktop/build-lib-mac.sh "$@"
+    ;;
+  desktop-windows)
+    scripts/desktop/build-lib-windows.sh "$@"
+    ;;
+  ios)
+    nix build '.#hydraJobs.aarch64-darwin."aarch64-darwin-ios:lib:simplex-chat"'
+    cp result/pkg-ios-aarch64-swift-json.zip "$HOME/Downloads/"
+    scripts/ios/prepare.sh
+    ;;
+  *)
+    printf 'usage: %s <android-aarch64|android-armv7a|desktop-linux|desktop-mac|desktop-windows|ios>\n' "$0"
+    exit 1
+    ;;
+esac