Add MathArena evaluation result for aime/aime_2026

This PR adds a new MathArena evaluation result so it can be indexed on the model leaderboard page.

Model: stepfun-ai/Step-3.5-Flash
Competition dataset id: MathArena/aime_2026
Score: 96.67
Result file: .eval_results/MathArena--aime_2026.yaml

The results are the same as the ones displayed on [our webpage](https://matharena.ai/?view=problem&comp=aime--aime_2026).

Note: this is an experimental feature, we are currently trying to make this work as smooth as possible.

.eval_results/MathArena--aime_2026.yaml

@@ -0,0 +1,8 @@
+- dataset:
+    id: MathArena/aime_2026
+    task_id: aime--aime_2026
+  value: 96.67
+  date: '2026-02-16'
+  source:
+    url: https://matharena.ai/?comp=aime--aime_2026
+    name: Official MathArena Evaluation