feat(course-registry): add enroll_many batch enrollment operation

[Degentle12]

Problem

Course-registry has only single-learner enrollment. A 100-learner cohort onboarding requires 100 separate transactions.

Solution

• enroll_many(env, admin, learners: Vec<Address>, id) -> Vec<bool> — Admin-authorized batch enrollment for cohort onboarding.
• Returns true for each newly-enrolled learner, false for already-enrolled (skipped).
• Does not abort on partial failure — each learner is processed independently.
• Validates course exists and is active before processing.
• 7 unit tests covering: success, partial duplicates, empty vec, all-already-enrolled, unauthorized admin, nonexistent course, and inactive course.

Closes #31