[17] Add retrieve_unenrolled and a refundable unenroll flow to course-registry #17

Problem Statement

course-registry::enroll (lines 158–177) writes DataKey::Progress(learner, id) = 0u32 but provides no inverse. A learner who enrolls by mistake is permanently enrolled — enroll panics with "Learner already enrolled" on retry — and the progress record persists in storage forever, paying rent indefinitely.

Why It Matters

• Storage rent on Soroban is real. Accumulating abandoned enrollments inflates state.
• Front-end UX: "I clicked enroll by accident" is unsolvable today.
• The codebase has no concept of learner-consented cancellation.

Technical Context

• DataKey::Progress(Address, u32) is per (learner, course).
• is_course_finished returns true for progress >= total_modules so unenrollment has to handle "in-progress vs complete."

Expected Outcome

• New unenroll(env, learner, id) function:
  • Learner auth required.
  • Removes Progress(learner, id) from storage.
  • Emits Unenrolled { learner, course_id } event.
  • Panics "Already completed" if progress >= course.total_modules (completion is irreversible).
• New is_enrolled(env, learner, id) -> bool view helper.
• Tests: success, double-unenroll, completed-course scenario, unauthorized.

Acceptance Criteria

• All unit + integration tests pass.
• Storage slot for unenrolled learners is removed (asserted via env.storage().persistent().has after call).

Implementation Notes

• Tie unenroll to badge status: if BadgeNFT has a badge for (learner, course), unenroll could be blocked since the credential exists.
• Consider a grace period to prevent griefing.

Files / Modules Affected

• contracts/course-registry/src/lib.rs
• contracts/course-registry/src/test.rs
• contracts/course-registry/README.md

Dependencies

None.

Difficulty

Easy.

Estimated Effort

3–4 hours.

Suggested Labels

feature, ux, P2, course-registry, good-first-issue