[MeABzZz]

What

This is the companion to the blvm-consensus PoW fix (fix(consensus): reject a block whose hash is above its target on connect).

test_verify_chain_level4_with_undo_logs builds its genesis and block 1 with bits: 0x0f00ffff and nonce 1 and 2. No hash meets that target. verifychain level 4 replays those blocks through connect with the ancestor lookup, and once connect checks proof of work it rejects them:

{"checked_blocks":2,"errors":["Block at height 0 invalid: Block hash does not meet its proof of work target"],"valid":false}

This PR gives the fixture regtest bits (0x207fffff) and grinds the nonce (mine_regtest, about 2 tries per header). Headers that meet their target pass with or without the consensus change. Only test code changes.

Evidence

• With the consensus PoW fix patched in, before this change: blockchain_rpc_tests had 3 passed and 1 failed, the failure shown above. It was the only failure in the full node suite (3018 passed).
• After this change: cargo test --test blockchain_rpc_tests has 4 passed and 0 failed.
• cargo fmt --all -- --check: clean.

Merge order

This can go in before or after the consensus PR. It must be in before node CI builds against a consensus that includes the PoW check.

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Review notes: merge order and open decisions (BLVM criticals set)

This PR is one of 7 fixing the four criticals from the BLVM three-lens review:
blvm-consensus#22 (undo log), blvm-consensus#23 (PoW vs target), blvm-node#32 (PoW test fixture), blvm-primitives#2 (min chainwork), blvm-node#31 (assume-valid), blvm-secp256k1#1 (no CUDA by default), blvm#10 (release build + no-CUDA check).

Merge order

1. blvm-node#32 before blvm-consensus#23, otherwise node CI breaks (old fixture headers have no valid PoW).
2. After the blvm-primitives release with Improve error handling #2, regenerate the consensus golden file consensus_config_default.json, or its test fails.
3. After the blvm-secp256k1 release with Add RPC method examples #1, run cargo update -p blvm-secp256k1 in blvm, then cut a release with blvm#10. v0.1.63 and v0.1.64 stay broken (need libcudart) until then.

Decisions for the reviewer

• PoW scope (fix(storage): detect signet_<hash> directories and respect network arg #23): checked on every node path; library/test calls that skip the difficulty lookup stay unchecked. Checking always would match Core but means re-mining all test fixtures.
• Undo log (fix(detection): add testnet4 arms to detect_network and testnet3 alias #22): nodes already synced on old code have empty undo logs stored and need a resync before a deep reorg is safe. Put this in release notes.
• Assume-valid (fix(node): anchor assume-valid per network with Core's hash and minimum work #31): BLVM rejects any block at height 938343 with a different hash, so it acts as a hard checkpoint (Core never does this). Kept as-is; testnet4 stays fully verified.

Known, not fixed here: -assumevalid=<hash> CLI parses the hash in the wrong byte order; the GHCR image ships a different binary (blvm 0.1.0).