feat: seamless rollovers between bond and stx-only staking positions

[brice-stacks]

A pox-5 staker can now move directly from one staking position to the
next with no gap in either signer participation or the STX lock:

• bond N -> bond N+6 (or any later non-overlapping bond) via
  register-for-bond
• bond N -> STX-only stake via stake
• STX-only stake -> bond via register-for-bond

The STX lock is carried forward in place (amount may go up or down,
unlock height is rescheduled) rather than released and re-acquired.
The bond's custodied sBTC is rolled forward via a net transfer:
equal amounts move zero sBTC, larger pulls only the delta from the
staker, smaller refunds the delta back. Rolling out of a bond into
stx-only (or any path where the new position has no sBTC) refunds
the bond's full sBTC custody.

This change also replaces my previous PRs #7237 and #7238.

The interesting commits are 957b509 and e313078

[brice-stacks]

I'll merge the latest pox-wf-integration into this, and then also move the integration test after I finish that change in #7223.

[coveralls]

Coverage Report for CI Build 26598194932

Coverage decreased (-0.2%) to 85.794%

Details

• Coverage decreased (-0.2%) from the base build.
• Patch coverage: 74 uncovered changes across 4 files (353 of 427 lines covered, 82.67%).
• 4987 coverage regressions across 77 files.

Uncovered Changes

File	Changed	Covered	%
stacks-node/src/burnchains/bitcoin_regtest_controller.rs	37	0	0.0%
stacks-node/src/burnchains/rpc/bitcoin_rpc_client/test_utils.rs	20	0	0.0%
clarity/src/vm/database/structures.rs	41	29	70.73%
pox-locking/src/pox_5.rs	329	324	98.48%

Coverage Regressions

4987 previously-covered lines in 77 files lost coverage.

Top 10 Files by Coverage Loss	Lines Losing Coverage	Coverage
stackslib/src/chainstate/burn/db/sortdb.rs	630	90.21%
stackslib/src/config/mod.rs	424	69.14%
stackslib/src/chainstate/stacks/boot/mod.rs	248	94.21%
clarity/src/vm/database/structures.rs	235	77.3%
stackslib/src/chainstate/stacks/miner.rs	224	83.06%
clarity/src/vm/database/clarity_db.rs	218	80.9%
stackslib/src/burnchains/burnchain.rs	209	72.01%
stackslib/src/chainstate/nakamoto/signer_set.rs	199	69.19%
stackslib/src/chainstate/stacks/db/transactions.rs	190	97.0%
stackslib/src/core/mempool.rs	181	86.44%

---

Coverage Stats

Relevant Lines:	222683
Covered Lines:	191048
Line Coverage:	85.79%
Coverage Strength:	17649171.54 hits per line

---

💛 - Coveralls

[brice-stacks]

This is built on top of #7223, and so includes those changes as well. The diff will shrink once that is merged.

[brice-stacks]

hmmm... looks like I may have messed something up with the last merge, causing errors in the new integration tests. Fixing now.

[brice-stacks]

My new test, check_pox_5_register_for_second_bond_no_downtime takes a really long time to run, because it needs to mine through 6 bond periods, so it's likely to always time out. If anyone has suggestions for shortening it, I'm all ears. I was unsuccessful in my attempts so far to shorten the reward phases, but I think that should be possible, and would help some.