Phase 6: advanced reports as SQL, relational algebra and stored routines
sql/reports.sql adds five reporting routines over the project schema:
pass rate per subject and semester, student dossier, top student per
major, busiest professor per semester and semester-over-semester growth.
Top-1-per-group is done with NOT EXISTS so it maps directly onto a
relational-algebra difference, and payments are joined through
enrolled_semesters per the normalized model from phase 5.
docs/Ph6.md is the wiki page: every report with its SQL, relational
algebra, stored routine and the output it produces on data_load.sql.
