fix: signature order check

[brice-stacks]

The last_index variable in verify_signer_signatures was only updated on the first loop iteration (inside an else branch), so subsequent signatures were checked against the first signer's index rather than the previous one. This allowed out-of-order sequences like [0, 2, 1] to pass validation. Move the assignment outside the conditional so it updates every iteration, and add a regression test.

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

[BowTiedRadone]

@brice-stacks This fixes #6907.

[coveralls]

Pull Request Test Coverage Report for Build 23526303949

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 158 unchanged lines in 34 files lost coverage.
• Overall coverage decreased (-0.02%) to 85.692%

---

Files with Coverage Reduction	New Missed Lines	%
libsigner/src/session.rs	1	65.41%
stacks-common/src/deps_common/bitcoin/util/hash.rs	1	81.82%
stacks-common/src/util/secp256r1.rs	1	90.24%
stackslib/src/burnchains/mod.rs	1	83.9%
stackslib/src/chainstate/stacks/index/node.rs	1	86.61%
stackslib/src/chainstate/stacks/miner.rs	1	83.48%
stackslib/src/clarity_vm/clarity.rs	1	93.7%
stackslib/src/net/api/poststackerdbchunk.rs	1	82.49%
stackslib/src/net/download/nakamoto/download_state_machine.rs	1	90.57%
stacks-signer/src/monitoring/server.rs	1	57.78%

Totals
Change from base Build 23400582460:	-0.02%
Covered Lines:	186535
Relevant Lines:	217681

---

💛 - Coveralls

[federico-stacks]

lgtm!

just a small question about the test configuration

[brice-stacks]

NOTE: Don't merge this until issue of old blocks is resolved.