Transmission #031: Progression math and StatId (implementation Step 1)

Implemented Step 1 of the stats/achievements plan: Helerion.Progression.StatId (seven stats), ProgressionMath.NextPowerOfTenAbove (strict next 1, 10, 100, … with NaN/∞ guard), and Edit Mode NUnit coverage under Assets/Tests/Editor/. The implementation plan checklist marks Step 1 complete. Unity batch tests were not executed here because the project was locked by another editor instance; run Edit Mode tests locally when the project is closed elsewhere.