BIP-0374: fix incorrect bit index and modernize CSV reader usage in test vector scripts

- **Corrected a logic bug in `gen_test_vectors.py`** Previously, the message tampering logic mistakenly used the proof damage index (`proof_damage_pos`) instead of the correct message index (`msg_damage_pos`). This could result in ineffective or misleading bit-flip tests. Now fixed to use the correct index for tampering the message. - **Modernized usage of `csv.reader` in `run_test_vectors.py`** Replaced the outdated `reader.__next__()` with the idiomatic and Python 3–safe `next(rea

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