Fix Docker base image tag and sync success_criteria across files

- Dockerfile: python:3.11-slim -> python:3.11.9-slim-bookworm
(specific patch+variant tag avoids manifest digest cache misses
in the validator's Docker registry)
- openenv.yaml: easy success_criteria 0.55 -> 0.62 (matches grader)
- server.py: medium description 0.45 -> 0.50 (matches _grade() check)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.11-slim
+FROM python:3.11.9-slim-bookworm
 
 # HF Spaces runs as a non-root user — create it early
 RUN useradd -m -u 1000 appuser

openenv.yaml

@@ -70,8 +70,9 @@ tasks:
     difficulty: easy
     description: >
       Clean dataset (flip_y=0.05), budget=300, max_steps=15.
-      Agent must reach validation performance > 0.55.
-    success_criteria: "current_performance > 0.55"
+      Agent must reach validation performance > 0.62.
+      Score is normalized over range [0.55, 0.75].
+    success_criteria: "current_performance > 0.62"
 
   - id: medium
     difficulty: medium

server.py

@@ -93,9 +93,9 @@ TASKS = {
         "description": (
             "High noise (flip_y=0.25), budget=150, max_steps=12. "
             "Agent must reach performance > 0.52 while keeping average "
-            "noise selection rate below 0.45. Uncertainty-only strategies fail."
+            "noise selection rate below 0.50. Uncertainty-only strategies fail."
         ),
-        "success_criteria": "current_performance > 0.52 AND avg noise_ratio < 0.45",
+        "success_criteria": "current_performance > 0.52 AND avg noise_ratio < 0.50",
         "cfg_overrides": {
             "data":           {"flip_y": 0.25},
             "budget":         150,