[WIP] - build: point at testnet4-aware bitcoinj / bitcoinj-thin

[jeremy-then]

Description

Dependency-only change: bump bitcoinj-core to 0.15.6-rsk-6 and bitcoinj-thin to 0.14.4-rsk-19
(plus the matching verification-metadata.xml entries). Those versions add the Bitcoin testnet4
network parameters, which the federator needs on both sides:

• the federator's full-bitcoinj SPV wallet (its own peer connection to the Bitcoin network), and
• the embedded rskj bridge (bitcoinj-thin) that validates the headers/txs the federator relays.

No powpeg code change is required for testnet4, the federator already builds its federation as a
segwit P2SH-P2WSH-ERP federation, which is exactly what the testnet2 genesis federation uses.

Motivation and Context

RSK is introducing a testnet2 network whose peg is backed by Bitcoin testnet4 (BIP-94, the successor
to testnet3). For a federator to operate that peg it must understand testnet4 (network magic, genesis,
BIP-94 difficulty rules) on both its SPV wallet and the embedded bridge. This PR pulls in that support via
the dependency bump. Companion PRs:

• rskj (testnet2 network + bridge/federation constants): rsksmart/rskj
• bitcoinj (full): rsksmart/bitcoinj — 0.15.6-rsk-6
• bitcoinj-thin: rsksmart/bitcoinj-thin — 0.14.4-rsk-19

How Has This Been Tested?

Built the full stack locally (composite build: powpeg-node + rskj + the testnet4-aware bitcoinj/bitcoinj-thin)
and ran a single self-mined federator on testnet2 against a real, fully-synced Bitcoin testnet4 node
(bitcoind v31). Exercised the federator-specific paths end-to-end:

• SPV wallet — the federator's full-bitcoinj wallet connected to a testnet4 peer and validated the
  testnet4 header chain (BIP-94) with no verification errors.
• Peg-in — BtcToRskClient detected a deposit to the federation, built the SPV proof, and relayed it via
  registerBtcTransaction; the bridge credited RBTC 1:1.
• Peg-out — BtcReleaseClient signed the segwit (P2SH-P2WSH-ERP) release with the BIP-143 sighash and
  broadcast it to testnet4; the confirmed release's change output was re-registered to the federation.
• Header relay — relayed the bridge's BTC header chain from testnet4 genesis to the tip
  (~143,800 headers) with zero verification failures.

Note: this is a dependency bump with no powpeg code change, so no new unit tests were added; validation was
end-to-end/integration as above.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• Tests for the changes have been added (for bug fixes / features)
• Requires Activation Code (Hard Fork)

• Other information:

  🚧 WIP / Draft — blocked on dependency releases. This branch references bitcoinj 0.15.6-rsk-6 and
  bitcoinj-thin 0.14.4-rsk-19, which are not published yet (pending review + security review + tagged
  release of the companion PRs). Until they are published, CI will fail to resolve the dependencies —
  expected. Once released, this branch builds as-is.

  • gradle/verification-metadata.xml uses trust rules for the two unpublished artifacts; they should be
    tightened to checksummed <component> entries once the artifacts are published.
  • Federator configuration (keyfiles/signers, the testnet2 overlay, BTC peers, bootnodes) is out of
    scope here and handled by ops — powpeg needs no code change for testnet4.

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[marcos-iov]

Replaced by #654