test: add PoX-5 support for TestChainstate

[francesco-stacks]

Description

Builds on top of #7507

Update TestChainstate to mine through Epoch 4.0 into PoX-5 cycles (and on into 4.1). Previously, any test crossing a reward cycle into Epoch 4.0 failed, because the harness only knew how to stack via pox-4.

It's unfortunately a lot of code, but 3-4k lines of changes are just the MARF hashes of the snapshots.

• setup_pox_5_signers: idempotent signer enrollment (signer-manager contract per key, key registration, staking), run automatically when advancing into Epoch 4.0+.
• Epoch fixtures place the 4.0/PoX-5 boundary at a "safe" reward-cycle offset and reject invalid schedules.

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). If this PR breaks
  anything for node operators or users, or requires them to manually do
  anything (such as adjust a setting), use the breaking category.
• 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

[Copilot]

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[coveralls]

Coverage Report for CI Build 32149491576

Coverage increased (+0.01%) to 86.578%

Details

• Coverage increased (+0.01%) from the base build.
• Patch coverage: Could not be determined — this PR's diff is too large for GitHub to return (406 error at GitHub).
• 326 coverage regressions across 32 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

326 previously-covered lines in 32 files lost coverage.

Top 10 Files by Coverage Loss	Lines Losing Coverage	Coverage
stackslib/src/chainstate/stacks/boot/mod.rs	212	94.38%
stackslib/src/net/p2p.rs	17	74.14%
stackslib/src/net/neighbors/comms.rs	10	77.25%
stacks-signer/src/v0/signer.rs	10	87.8%
stacks-node/src/neon_node.rs	8	83.27%
stacks-node/src/nakamoto_node/miner.rs	7	86.49%
stackslib/src/net/stackerdb/config.rs	6	81.45%
stacks-signer/src/chainstate/v2.rs	6	88.38%
stackslib/src/net/unsolicited.rs	5	82.4%
clarity/src/vm/functions/bitcoin_madhouse.rs	4	82.26%

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Coverage Stats

Relevant Lines:	233020
Covered Lines:	201744
Line Coverage:	86.58%
Coverage Strength:	20053564.23 hits per line

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💛 - Coveralls

[brice-stacks]

nice! just one tiny nit.

[brice-stacks]

nit: _e