Besu 26.6.1 Defaults Missing Blocks to Latest

Besu 26.6.1 RPC now defaults missing block params to "latest" for six methods. PoW and several flags face deprecation. Bug fixes address engine_getBlobsV2 sync errors and WebSocket stalls. Added eth_getTransactionBySenderAndNonce, with optional sender+nonce indexing. Checksums: zip 0adcf27c, tar.gz 5bf53666.