[aaronb-stacks]

From Epoch 4.1, PoX-5 signer-set computation derives the per-cycle sBTC waterfall recipient from the sBTC registry's current-signer-set and current-signature-threshold (via get-current-signer-data) as an sBTC v2 Taproot script-path multisig deposit, instead of from the registry's WSTS aggregate public key (get-current-aggregate-pubkey), which stops being a public key once sBTC rotates to v2 signing (stacks-sbtc/sbtc#2138).

The epoch of the block computing the reward set selects the format, so the block-commit output switches at the first reward cycle whose prepare phase begins in Epoch 4.1. The Epoch 4.0 path is unchanged.

This PR adds:

• the BIP-32 signer-key derivation and multi_a script, v2 deposit prefix and v2 NUMS constant to chainstate::stacks::sbtc
• fixtures pinned to sBTC's published signer-UTXO scriptPubKeys
• a consensus test walking the reward sets across the 4.0 -> 4.1 boundary
• a bitcoind integration test for the block-commit switch.
• a contrib binary for generating test fixtures from the sbtc crate directly
• a CI job for verifying that the committed test fixtures match the output of the generator exactly.

[Copilot]

🔵 Needs a closer look

The lockfile lacks the newly required hmac 0.12 package entry, so CI and release builds using --locked will fail until the complete workspace lockfile is regenerated and committed.

No open issues 🎉

What changed in this PR

This PR updates PoX-5 reward-set computation to derive sBTC recipients from Taproot multisig signer data in Epoch 4.1, while preserving the Epoch 4.0 aggregate-key path.

Changes:

• Adds epoch-selected v2 deposit derivation, including signer-key derivation and multisig scripts.
• Adds reference fixtures, property tests, and consensus and bitcoind transition tests.
• Adds a pinned upstream fixture generator and CI reproducibility checks.

File	Description
stackslib/​src/​net/​api/​getpoxinfo.rs	Updates registry-method comments only.
stackslib/​src/​core/​mod.rs	Adds v2 internal-key and signer-limit constants.
stackslib/​src/​config/​mod.rs	Documents epoch-dependent registry reads.
stackslib/​src/​chainstate/​tests/​mod.rs	Extends registry stubs with signer data.
stackslib/​src/​chainstate/​tests/​consensus.rs	Tests recipient selection across epoch activation.
stackslib/​src/​chainstate/​stacks/​sbtc.rs	Implements and tests v2 deposit derivation.
stackslib/​src/​chainstate/​stacks/​fixtures/​sbtc-v2.json	Adds upstream-derived reference fixtures.
stackslib/​src/​chainstate/​nakamoto/​signer_set.rs	Selects recipient derivation by computing epoch.
stackslib/​Cargo.toml	Adds HMAC for signer-key derivation.
stacks-node/​src/​tests/​signer/​v0/​mod.rs	Extends integration-test registry stubs.
stacks-node/​src/​tests/​signer/​v0/​epoch_4_0_waterfall.rs	Adds a bitcoind recipient-switch test.
stacks-node/​src/​tests/​nakamoto_integrations.rs	Adds an Epoch 4.1 activation helper.
stacks-common/​src/​types/​tests.rs	Tests deposit-format activation.
stacks-common/​src/​types/​mod.rs	Defines epoch-specific deposit formats.
contrib/​tools/​sbtc-fixtures/​src/​main.rs	Generates fixtures using upstream sBTC.
contrib/​tools/​sbtc-fixtures/​README.md	Documents generation and reproducibility checks.
contrib/​tools/​sbtc-fixtures/​inputs.json	Defines seven fixture input cases.
contrib/​tools/​sbtc-fixtures/​Cargo.toml	Defines the standalone generator and dependencies.
contrib/​tools/​sbtc-fixtures/​Cargo.lock	Locks generator dependencies.
changelog.d/​epoch-4-1-sbtc-taproot-multisig.changed	Describes the recipient-format transition.
Cargo.lock	Adds the workspace HMAC dependency reference.
.github/​workflows/​check-sbtc-fixtures.yml	Checks fixture regeneration byte for byte.

_{Review effort: Balanced}