feat(consensus): regression test for StarfishSpeed phantom-ack invariant

[VorobyevIlya]

Description of change

Adds a failing test that catches a safety-invariant violation in StarfishSpeed Optimistic commit: strong-vote bytes computed against one leader get credited as phantom acks for a different canonical leader after a schedule rotation, leading to transactions being committed without 2f+1 stake actually having their data. Also extracts Core::compute_strong_vote_for as a pub(crate) static helper so the test drives bytes through production code rather than fixtures.

How the change has been tested

• Basic tests (linting, compilation, formatting, unit/integration tests)
• Patch-specific tests (correctness, functionality coverage)
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes