mppread: a .mpp with a deleted resource is refused (its calendar record is not recognised) #508

Found while working #445 (review r1, M5). When a resource is deleted in Microsoft Project, the saved .mpp keeps the deleted resource's calendar as a record the current calendar decoder does not recognise ("calendar record 7" in the generated corpus/mpp/assnbaseline/a3-deleted-rows.mpp, from corpus/tools/gen_mpp_assignment_baseline_cases.py). mppread::caldecode refuses the record, so project_from_mpp refuses the whole file: any plan that ever had a resource deleted cannot be opened. This was already true on main before #445; #445 only tests its resource/assignment decoders on that file directly.

Fix: identify how Project marks a deleted resource's calendar in TBkndCal (validate against the a3 case's Project XML, which omits it) and skip it instead of refusing. Test: project_from_mpp opens a3-deleted-rows.mpp and its calendars match Project's XML.

Source: #445, PR #507
Severity: minor; origin: review r1