engine: stage presigned objects as .dat; task entry reports the failing step and HTTP status

Through Cloudflare, signed requests for keys ending .tar/.gz/.zip/.bin reached
RustFS altered and failed with 403 (cf-cache-status BYPASS); the same signed
request straight to RustFS on the LAN returned 200. A Cloudflare cache-bypass
rule on files.because-security.com fixes it (all extensions 200, DYNAMIC); the
engine executors keep .dat as a safeguard, the plane keeps output.tar.
Implemented with Codex.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

benchmarks/learn/engine_trace_k8s.py

@@ -12,7 +12,7 @@ import time
 import uuid
 from datetime import timedelta
 
-from openultrasast.plane.memory import S3Store, open_store
+from openultrasast.plane.memory import PRESIGNED_ARCHIVE_SUFFIX, S3Store, open_store
 
 IMAGE = "ghcr.io/norandom/openultrasast:2.0.1"
 BOOTSTRAP = r"""
@@ -164,7 +164,7 @@ class Kubernetes:
         name = "engine-" + uuid.uuid4().hex[:24]
         pin_key = hashlib.sha256(json.dumps([pin.get("repo"), pin.get("pin")]).encode()).hexdigest()
         prefix = f"engine-jobs/{self.run}/{pin_key}/"
-        keys = [prefix + file for file in ("source.tar", "analyzer.tar", "result.tar")]
+        keys = [prefix + name + PRESIGNED_ARCHIVE_SUFFIX for name in ("source", "analyzer", "result")]
         expiry = timedelta(seconds=self.args.deadline + 900)
         node = None
         try: