Improve self-paced learner journey

[bofandra]

Summary

Improve the open-course learner experience without changing the OOP curriculum or its two-reference boundary.

Changes

• add LEARNER_PROGRESS.md for Module 0–16 progress, evidence, reflections, and capstone checkpoints;
• add READING_GUIDE.md with the official free OpenStax access path and lawful-use guidance for the Inggriani Liem Diktat;
• link both learner tools from README.md, START_HERE.md, and SELF_PACED_GUIDE.md;
• extend scripts/validate_course.py so these learner-facing assets remain part of publication validation;
• document the change in CHANGELOG.md.

Intent

Reduce two remaining sources of learner friction:

1. uncertainty about what to do after finishing each module;
2. uncertainty about how to access/use the formal references without making an unofficial copy a course dependency.

No learning outcomes, assessment scope, or formal references are changed.