dual_decomposition: extend strengthened conjecture table to n=20

n=19 (21,138 col., 68s) and n=20 (107,874 col., 361s) both pass.
New total for clauses (1)-(4) over n<=20: 142,812/142,812.

Also bump max_n in check_conj_3_8_scaled.py default to 20 (was 18) and
time_budget_per_n to 7200s.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

papers/dual_decomposition_minimal_counterexamples/paper.tex

@@ -647,12 +647,14 @@ $(F, e_1, e_2)$ whose accompanying $f_n$ satisfies clause~(4) (see
 \begin{tabular}{r|r|r|r|l}
 $n$ & \#tri & \#col.\ tested & \#sat. & status \\
 \hline
-$14$ & $1$  & $216$       & $216$     & all pass \\
-$16$ & $3$  & $864$       & $864$     & all pass \\
-$17$ & $4$  & $4{,}650$   & $4{,}650$ & all pass \\
-$18$ & $12$ & $8{,}070$   & $8{,}070$ & all pass \\
+$14$ & $1$  & $216$       & $216$       & all pass \\
+$16$ & $3$  & $864$       & $864$       & all pass \\
+$17$ & $4$  & $4{,}650$   & $4{,}650$   & all pass \\
+$18$ & $12$ & $8{,}070$   & $8{,}070$   & all pass \\
+$19$ & $23$ & $21{,}138$  & $21{,}138$  & all pass \\
+$20$ & $73$ & $107{,}874$ & $107{,}874$ & all pass \\
 \hline
-total ($n \le 18$) & $20$ & $13{,}800$ & $13{,}800$ & \\
+total ($n \le 20$) & $116$ & $142{,}812$ & $142{,}812$ & \\
 \end{tabular}
 \end{center}
 \noindent A subtlety: only about half of the