Add direct coverage for serialization helpers and tree boundaries

[Copilot]

• Inspect CI workflow runs and collect compiler error logs
• Reproduce the compiler error locally
• Apply minimal code fix for the reported compiler issue
• Run targeted tests/build to verify the fix
• Run final validation (code review + CodeQL) and report back

[Copilot]

Pull request overview

This PR adds a dedicated “coverage” test binary to exercise low-level serialization helpers and boundary behaviors, while tightening PathT equality semantics and centralizing validation for partial tree serialization ranges.

Changes:

• Add test/coverage.cpp to directly cover serialization helpers, PathT metadata/deserialization/equality, partial-serialization boundary cases, and TreeT move/self-assignment.
• Update PathT equality to include leaf/max-index metadata and make operator!= const.
• Centralize partial range validation for TreeT::serialise(from,to) and TreeT::serialised_size(from,to), including explicit empty-tree rejection.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/coverage.cpp	New targeted coverage test executable covering helper functions and boundary/error paths.
test/CMakeLists.txt	Registers the new coverage test target with the existing test harness.
merklecpp.h	Refines PathT equality/inequality semantics and factors partial-range validation into a shared helper.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[achamayou]

@copilot look at the compiler errors and fix them

[Copilot]

@copilot look at the compiler errors and fix them

Fixed. I investigated the CI logs, resolved the compiler/clippy-style errors in test/coverage.cpp, and fixed the crash by hardening TreeT move/copy assignment ownership handling. The fix is in commit 3f1edbb (plus earlier prep in 05797c0), and local + CI-like validation now pass.