P2867R2 Remove deprecated strstreams from C++26

As no other polls for Tokyo touch the table of library headers,
I rebalanced the table columns.

Slight tweak to Annex C drafting when importing header units
to be consistent with previous wording --- the intent in LWG
was to match precedent.

Author: AlisdairM

source/compatibility.tex

@@ -190,6 +190,25 @@
 \item \tcode{little_endian}.
 \end{itemize}
 
+\nodiffref
+\change
+Remove header \libnoheader{strstream} and all its contents.
+\rationale
+The header has been deprecated since the original \Cpp{} standard; the
+\libheader{spanstream} header provides an updated, safer facility.
+Ongoing support is at implementer's discretion,
+exercising freedoms granted by \ref{zombie.names}.
+\effect
+A valid \CppXXIII{} program \tcode{\#include}-ing the header or importing the
+header unit may become ill-formed. Code that uses any of the following classes
+by importing one of the standard library modules may become ill-formed:
+\begin{itemize}
+\item \tcode{istrstream}
+\item \tcode{ostrstream}
+\item \tcode{strstream}
+\item \tcode{strstreambuf}
+\end{itemize}
+
 \rSec1[diff.cpp20]{\Cpp{} and ISO \CppXX{}}
 
 \rSec2[diff.cpp20.general]{General}