fix(v2/hcore): correct fmt usage and formatting issues

[nisemenov]

Problem

Some fmt usages may lead to incorrect formatting or runtime issues:

• fmt.Sprint used with formatting directives (%d)
• fmt.Printf called with non-constant format strings

Solution

• Replace fmt.Sprint with fmt.Sprintf where needed
• Use explicit format strings in fmt.Printf / log.Printf

These changes improve correctness and avoid potential formatting bugs.