hidden_ref: invalid numeric options silently select defaults #240

In hidden_ref, -c garbage becomes zero through atoi and then becomes the token count. -t garbage becomes zero and keeps the library's default threads. With otherwise valid inputs, the tool writes a completed reference set instead of rejecting invalid options. Replace atoi/atoll with checked parsing that consumes the full argument and verifies its range. Add rejection cases for nonnumeric text and trailing junk.

Evidence (at the base of #236):

• tools/ref/hidden_ref.cpp:130
• tools/ref/hidden_ref.cpp:150
• tools/ref/hidden_ref.cpp:174

_{Filed by Shingler, the review bot, while reviewing #236; the defect is outside that pull request.}