Learner experience and release readiness

[bofandra]

Post-audit learner-experience cleanup.

Changes:

• add a first-visit "Start in 10 minutes" path to README;
• make START_HERE immediately actionable with direct readiness/Module 0/Module 1 links;
• align self-assessment checkpoints with the audited distinctions for invariants, transition rules, associations, and runtime validation;
• fix Instructor Guide wording so optional activity durations are not confused with a fixed schedule;
• strengthen the validator so key onboarding links cannot silently disappear;
• document the onboarding-navigation regression check in EXECUTION_AUDIT.

This PR changes navigation/readiness only; it does not expand course scope.