Improve placeholder scanning for panic macros

[oferchen]

Summary

• extend the no-placeholders scan to track panic! invocations across multiple lines and handle delimiter comments
• ensure placeholder words inside panic! blocks (even after comments) trigger a violation
• add coverage for multi-line panic! invocations in the xtask test suite

Testing

• cargo test -p xtask

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https://chatgpt.com/codex/tasks/task_e_69055719b8b48323955538d6840254c9