Function patch_seq_string_substring 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn patch_seq_string_substring(
    stack: Stack,
) -> Stack

Extract a substring using character indices

Stack effect: ( str start len – str )

Arguments:

• str: The source string
• start: Starting character index
• len: Number of characters to extract

Edge cases:

• Start beyond end: returns empty string
• Length extends past end: clamps to available

Safety

Stack must have String, Int, Int on top