fix: revert prebuilt to 12 — topk .so not in base image build env

corex_moe_topk_softmax.so compiled on dev machine but docker build
env may differ. Revert to 12 prebuilt to unblock submission.
max_completion_tokens + n=2 fixes are the priority.

qwen3_6_scripts/install_prebuilt_corex.sh

@@ -16,8 +16,8 @@ MANIFEST=${BUNDLE_DIR}/SHA256SUMS
 }
 
 mapfile -t artifacts < <(awk '{print $2}' "$MANIFEST")
-[[ "${#artifacts[@]}" -eq 13 ]] || {
-    printf 'expected 13 prebuilt CoreX artifacts, found %s\n' \
+[[ "${#artifacts[@]}" -eq 12 ]] || {
+    printf 'expected 12 prebuilt CoreX artifacts, found %s\n' \
         "${#artifacts[@]}" >&2
     exit 2
 }

qwen3_6_scripts/prebuilt/corex-3.2.3-ivcore10/SHA256SUMS

@@ -10,4 +10,3 @@ ec2d11fa82d9d0816a6da53e62605e962786fa20ecd5f62e50f9d43087fc4d67  corex_gdn_gate
 d26f2fa39c3921a95793786601e90cf6ebadd06f1d752af541bf82c21acbc1c9  corex_moe_exact_reduce.so
 50b0b44c1da779bb2c03419ed549aee9bb922d1f9bab8b7f11a3d91cca0d21c3  corex_moe_weight_gather.so
 e944ec0528ed9b6cb74518de3c57e3730543a7bdebc872f993bfdc8424f13e6b  corex_paged_kv_gather.so
-c3208c8e0c13f54dbe22a9cfc88bdc6ab040e920d6cae4bc0ecf7087880795f3  corex_moe_topk_softmax.so