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/experiments/check_conj_3_8_scaled.py

@@ -359,7 +359,7 @@ def check_clause_4_kempe_part(H, edges, col_list, named, witness, a, b, c,
     return count == 1
 
 
-def main(max_n=18, time_budget_per_n=3600):
+def main(max_n=20, time_budget_per_n=7200):
     rows = []
     for n in range(12, max_n + 1):
         start = time.time()