docs(design): the build database of #636 and the two defects on the way to the latest xlings

The record of the analysis, the plan and its self-review: xlings stages a
hookless package from the shared download directory; a POSIX redirect in the
vendored-xlings probe reaches cmd.exe on Windows; and mcpp emits an S1 build
database from the plan without writing into the project.

Author: speak-agent