fix: validate CbTx chainlock height diff

[thepastaclaw]

Issue being fixed or feature implemented

CheckCbTxBestChainlock derived an ancestor height from bestCLHeightDiff
without first ensuring the derived height was in range. This PR adds an
explicit consensus rejection for out-of-range CbTx chainlock height diffs before
ancestor lookup.

What was done?

• Validate bestCLHeightDiff before deriving and using the referenced chainlock
  ancestor height.
• Return bad-cbtx-cldiff for out-of-range values.
• Add a defensive null check around the ancestor lookup.
• Add focused unit coverage for boundary values in a new evo_cbtx_tests suite.

How Has This Been Tested?

Tested locally on macOS arm64:

cd src && make -j6 test/test_dash
./test/test_dash --run_test=evo_cbtx_tests
./test/test_dash '--run_test=evo_*'

Results:

make -j6 test/test_dash: pass
./test/test_dash --run_test=evo_cbtx_tests: 1/1 pass
./test/test_dash '--run_test=evo_*': 38/38 pass

Pre-PR review gate:

code-review dashpay/dash upstream/develop review/fix-cbtx-chainlock-height-bound "Pre-PR review: validate CbTx bestCLHeightDiff range before ancestor lookup and add focused regression tests"

Result: ship.

Breaking Changes

None expected. This only rejects malformed CbTx chainlock metadata that
references an invalid ancestor height.

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone

[thepastaclaw]

@coderabbitai review

[coderabbitai]

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Review finished.

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[github-actions]

⚠️ Potential Merge Conflicts Detected

This PR has potential conflicts with the following open PRs:

• feat: add masternode reward shares #7340 - feat: add masternode reward shares

Please coordinate with the authors of these PRs to avoid merge conflicts.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 31f55f2e-117d-4bcf-be01-e2fb1bf61564

📥 Commits

Reviewing files that changed from the base of the PR and between 7fcc54a and 7312fff.

📒 Files selected for processing (4)

• src/Makefile.test.include
• src/evo/specialtxman.cpp
• src/evo/specialtxman.h
• src/test/evo_cbtx_tests.cpp

---

Walkthrough

This PR introduces stricter validation for the bestCLHeightDiff field in Dash coinbase transactions. The changes export a validation function that checks both the height difference bounds (rejecting values that would imply pre-genesis ancestors) and safely handles missing block indices during ancestor lookups. A unit test verifies rejection at boundary conditions, and the test is integrated into the build system.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Suggested reviewers

• UdjinM6
• PastaPastaPasta

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'fix: validate CbTx chainlock height diff' directly and concisely summarizes the main change: adding validation for the chainlock height difference field in CbTx.
Description check	✅ Passed	The pull request description comprehensively details the issue being fixed, specific changes made, testing performed, and breaking change assessment, all directly related to the changeset.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

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[thepastaclaw]

CI note: the only red item is predict_conflicts; the full build/test matrix is green. The predictor is flagging overlapping test/build files and Evo specialtx files against other open PRs (including #7340 and nearby queued work). I’m leaving this as a merge-sequencing/conflict-prediction blocker rather than an application/test failure for now.