feat: add support for testnet4

[brice-stacks]

Description

Applicable issues

• fixes #

Additional info (benefits, drawbacks, caveats)

Checklist

• Test coverage for new or modified code paths
• For new Clarity features or consensus changes, add property tests (see docs/property-testing.md)
• Changelog fragment(s) or "no changelog" label added (see changelog.d/README.md)
• Required documentation changes (e.g., rpc/openapi.yaml for RPC endpoints, event-dispatcher.md for new events)
• New clarity functions have corresponding PR in clarity-benchmarking repo

[coveralls]

Coverage Report for CI Build 26182114351

Warning

Build has drifted: This PR's base is out of sync with its target branch, so coverage data may include unrelated changes.
Quick fix: rebase this PR. Learn more →

Coverage decreased (-0.1%) to 85.573%

Details

• Coverage decreased (-0.1%) from the base build.
• Patch coverage: 41 uncovered changes across 6 files (14 of 55 lines covered, 25.45%).
• 5672 coverage regressions across 110 files.

Uncovered Changes

File	Changed	Covered	%
stackslib/src/burnchains/bitcoin/indexer.rs	19	0	0.0%
stackslib/src/burnchains/bitcoin/spv.rs	21	4	19.05%
stackslib/src/chainstate/coordinator/mod.rs	9	7	77.78%
stacks-node/src/main.rs	1	0	0.0%
stacks-node/src/node.rs	1	0	0.0%
stackslib/src/config/mod.rs	3	2	66.67%

Coverage Regressions

5672 previously-covered lines in 110 files lost coverage.

Top 10 Files by Coverage Loss	Lines Losing Coverage	Coverage
stackslib/src/config/mod.rs	376	68.99%
stackslib/src/chainstate/stacks/index/storage.rs	262	80.55%
stackslib/src/chainstate/stacks/miner.rs	259	83.06%
stackslib/src/chainstate/stacks/db/transactions.rs	222	97.16%
stackslib/src/net/inv/epoch2x.rs	221	79.49%
stackslib/src/net/chat.rs	201	93.01%
stackslib/src/chainstate/stacks/db/mod.rs	196	86.23%
stackslib/src/core/mempool.rs	193	86.44%
stackslib/src/chainstate/stacks/index/trie_sql.rs	190	69.7%
stackslib/src/chainstate/stacks/index/node.rs	159	87.41%

---

Coverage Stats

Relevant Lines:	220088
Covered Lines:	188335
Line Coverage:	85.57%
Coverage Strength:	18022239.1 hits per line

---

💛 - Coveralls

[CLAassistant]

All committers have signed the CLA.

[benjamin-stacks]

Side note: The Testnet config here is completely broken, as I discovered the other day. This should be nakamoto_testnet_default(), and a couple of other things (pox constants, first block, ...) also need to be changed.

There's also references in the code to an "old hiro testnet" that no longer exists.

Not sure that that matters for anything here, just felt like a good time to mention it.

[brice-stacks]

Putting this on hold until we decide if we really need to support testnet4.