plot: extend batch-scaling curves with saturated-throughput ceiling

Beyond a mapping's SIP capacity (16/C) the throughput plateaus: extra
users run in waves at the saturated rate, they are not un-runnable. Draw
a dashed horizontal extension at the ceiling so 1-kv-per-cube (cap 2)
reads as saturating, not stopping, at B=2. Caption updated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/report/1H-codesign-paper/sections/05-gqa.tex

@@ -271,10 +271,13 @@ long-context batch sweep is 2H work (\S\ref{sec:future}).
 \si{\micro\second}) versus the number of concurrent decode users on one SIP,
 each user on a disjoint CUBE group. Each mapping scales almost linearly with
 $B$ up to its SIP capacity $16/C$---\textsf{1-kv-per-cube} saturates at two
-users, \textsf{8-kv-per-cube} at sixteen. The dense mapping reaches the
-highest throughput; the spread mapping is latency-optimal but
-capacity-limited. Concurrent users overlap to within
-\SI{3}{}--\SI{5}{\percent} of the single-user latency.}
+users, \textsf{8-kv-per-cube} at sixteen. Solid segments are measured
+concurrent runs; the dashed extensions are the saturated-throughput ceiling
+beyond capacity, where further users run in waves at that sustained rate
+(so the SIP is full, not idle). The dense mapping reaches the highest
+ceiling; the spread mapping is latency-optimal but capacity-limited.
+Concurrent users overlap to within \SI{3}{}--\SI{5}{\percent} of the
+single-user latency.}
 \label{fig:gqa-batch}
 \end{figure}