Fix flaky opt_priokey -strict and cover binary exclusive scan

ASLR-dependent ConstEval seeding let E4 miss OOR counterexamples; use a
deterministic seed and force OOR key collisions. Add I9 (NB=12) for the
thermometer fallback path Greptile flagged.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/opt/opt_first_fit_alloc.ys

@@ -1171,3 +1171,42 @@ design -load postopt
 select -assert-min 1 w:*ffa_*
 design -reset
 log -pop
+
+# I9: exclusive with NB > max_therm_nb (8) — forces the binary saturating
+# Hillis–Steele fallback (emit_scan_exclusive_bin / emit_sat_add) instead of
+# the thermometer scan used by I1–I8.
+log -header "I9: exclusive binary-scan fallback N=16 NB=12 (equiv)"
+log -push
+design -reset
+read_verilog -sv <<EOF
+module top #(parameter N=16, NB=12, W=4) (
+  input  logic           mode,
+  input  logic [N-1:0]   req,
+  output logic [N*W-1:0] dsel_flat
+);
+  logic [N-1:0]  en = req & {N{mode}};
+  logic [W-1:0]  dsel [0:N-1];
+  logic [NB-1:0] taken;
+  logic [N-1:0]  done;
+  always_comb begin
+    for (int i=0;i<N;i++) dsel[i] = '0;
+    taken = '0; done = '0;
+    for (int i=0;i<N;i++)
+      if (en[i])
+        for (int j=0;j<NB;j++)
+          if (!taken[j] && !done[i]) begin
+            dsel[i] = W'(j); done[i] = 1'b1; taken[j] = 1'b1;
+          end
+  end
+  for (genvar g=0;g<N;g++) assign dsel_flat[g*W +: W] = dsel[g];
+endmodule
+EOF
+hierarchy -top top
+proc
+opt
+check -assert
+equiv_opt -assert opt_first_fit_alloc
+design -load postopt
+select -assert-min 1 w:*ffa_*
+design -reset
+log -pop