Behaviour for a fixture-session projection function, pluggable into mix raxol.harness.fixtures.bless.

The real journal-fold projection (roadmap unit T7, expected Raxol.Harness.Projection per 06-projection §7 open question 4) does not exist in code yet. Raxol.Harness.Fixture.Projectors.Identity is the trivial placeholder that keeps the bless task testable ahead of it; any module implementing this one callback can be swapped in via --projector.