Add addOptionalCourses, tests; reduce MAX_RESULTS

Introduce addOptionalCourses to generate valid combinations including optional courses and export it from generateCombinations.ts. Reduce MAX_RESULTS from 1_000 to 100 and add guards/limits in generateCombinationsOptimized (single-course slicing and empty-course early-return) to avoid runaway generation. Add extensive unit tests: new addOptionalCourses tests, expanded binaryMeetingTime tests (meetingTimesToBinaryMask, masksConflict, incrementIndex) and updated generateSchedules tests to exercise the locked+optional pipeline. These changes improve correctness for optional course handling and strengthen test coverage.

Author: YasushikoX