docs: the host-dependence rule is per axis; second person out of the reference doc

The design document argued a single boundary — "does it build and run" — for
every host dependency, and D15 followed it to "warn, do not refuse". §1.2 is
corrected to state the rule per axis: the toolchain is mcpp's contract and is
refused, the libraries a program links are the program's own and stay a
warning. §7, §9, §10, §11.7, §12 and the review record follow.

The measurement behind the earlier conclusion was right and stays in the
document — `[toolchain] system` does build a project using `import std`. What
was wrong was carrying it across an axis boundary, which is the failure this
document keeps finding from the other side.

Also drops a second-person sentence from docs/06 that check_docs_style.sh
refuses in a reference doc.

Author: speak-agent