ok  export headers exact
  ok  export lists all 3 batch students
  ok  Arabic name survives xlsx round-trip
  ok  main import writes 3 rows
  ok  grades stored
  ok  every grade write audited
  ok  identical file rejected
  ok  only changed row rewritten
  ok  audit has before/after
  ok  rejects: bad header
  ok  rejects: unknown student
  ok  rejects: duplicate student
  ok  rejects: mark out of range
  ok  rejects: mark AND code
  ok  rejects: bad code
  ok  rejects: broken encoding
  ok  failed imports wrote nothing
  ok  failed imports are logged too
  ok  second-round export = F(69) + Ab* only, not the B
  ok  second round rejects passed student
  ok  second round rejects codes
  ok  second-round marks stored
  ok  F(69) cleared by supplementary(75) → C* capped 2.00
  ok  untouched pass stays B
  ok  Ab* substitute takes real grade (90 aviation = A)

25 passed, 0 failed