Initialize unused1, unused2, and unused3 fields to zero in TransactionData to ensure consistent encoding/decoding behavior.

Background:
In commit a0c9fec, we updated TransactionData encoding/decoding and bumped the Transaction encoding version from 9 to 10. As part of this change, we renamed three fields to mark them as unused:

• largest_data_len → unused1
• largest_data_off → unused2
• largest_data_off_in_data_bl → unused3

The move constructor was also updated to stop setting these fields, leaving them uninitialized after move operations.

Problem:
This worked with existing tests because check-generated.sh reused struct instances, preserving stale values across encode/decode cycles. However, an upcoming test change will stop reusing instances and compare hexdumps of encoded/re-encoded values to verify consistency. Uninitialized fields cause these comparisons to fail due to garbage values.

Solution:
Initialize the unused fields to zero in the move constructor. This preserves existing behavior while ensuring consistent encoding. These fields can be removed entirely in a future change.

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