chore: import upstream snapshot with attribution

scripts/_common.sh

@@ -0,0 +1,50 @@
+#===============================================================================
+# Common shell utilities: logging, command checks, checksum verification.
+#
+# Sourced by: build.sh
+# Sourced globals: (none)
+# Modifies globals: (none)
+#===============================================================================
+
+check_command() {
+	if ! command -v "$1" &> /dev/null; then
+		echo "$1 not found"
+		return 1
+	else
+		echo "$1 found"
+		return 0
+	fi
+}
+
+section_header() {
+	echo -e "\033[1;36m--- $1 ---\033[0m"
+}
+
+section_footer() {
+	echo -e "\033[1;36m--- End $1 ---\033[0m"
+}
+
+verify_sha256() {
+	local file_path="$1"
+	local expected_hash="$2"
+	local label="${3:-file}"
+
+	if [[ -z $expected_hash ]]; then
+		echo "Warning: No SHA-256 hash for ${label}," \
+			'skipping verification' >&2
+		return 0
+	fi
+
+	echo "Verifying SHA-256 checksum for ${label}..."
+	local actual_hash _
+	read -r actual_hash _ < <(sha256sum "$file_path")
+
+	if [[ $actual_hash != "$expected_hash" ]]; then
+		echo "SHA-256 mismatch for ${label}!" >&2
+		echo "  Expected: $expected_hash" >&2
+		echo "  Actual:   $actual_hash" >&2
+		return 1
+	fi
+
+	echo "SHA-256 verified: ${label}"
+}