Add macOS x86_64 platform support

Platform detection now happens in framework/platform, sourced by both
sync.sh and the node shim. Uses shasum on macOS, sha256sum on Linux.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

framework/platform

@@ -0,0 +1,26 @@
+# shellcheck shell=bash
+
+# Detect platform (OS and architecture)
+
+os=$(uname -s | tr '[:upper:]' '[:lower:]')
+arch=$(uname -m)
+
+case "$os" in
+    linux) platform_os=linux ;;
+    darwin) platform_os=darwin ;;
+    *) echo "Unsupported OS: $os" >&2; exit 1 ;;
+esac
+
+case "$arch" in
+    x86_64) platform_arch=x86_64 ;;
+    *) echo "Unsupported architecture: $arch" >&2; exit 1 ;;
+esac
+
+platform="${platform_os}_${platform_arch}"
+
+# Platform-specific checksum command
+if [ "$platform_os" = "darwin" ]; then
+    sha256_check() { shasum -a 256 -c -; }
+else
+    sha256_check() { sha256sum -c -; }
+fi