Tighten assessment concept progression

[bofandra]

Summary

This PR tightens assessment sequencing without changing the formal course scope.

• removes premature use of Python's is operator from Module 2 exercises
• clarifies Module 5 mastery feedback so has-a is not automatically equated with strict composition
• aligns Module 3 self-assessment wording with the deliberate validation handoff to Module 4
• adds a validator guard for later-course syntax/mechanisms appearing too early in learner-facing exercises, quizzes, or assignments
• updates the execution audit and changelog

The goal is to preserve the intended progression: concept first, implementation mechanism when introduced, then assessment.