perf(push): drop per-entry cache fsync; let long runs profile themselves

The cache fsynced every payload and manifest -- two per entry, ~18 ms, and a
push publishes one entry per question (~22,000 on a WordPress plugin). A
read already rejects any entry whose manifest receipt, payload size or
payload SHA-256 does not check, so a torn write is a miss, never a wrong
answer; the fsyncs bought nothing the verification does not.

The broader problem: three quadratic walls in this replay were found one
per two-hour run, because all were Python bookkeeping between engine calls
and the only record was the engine log. `OUSAST_SAMPLE_PROFILE=<path>`
starts an in-process sampler that rewrites the hottest stacks every 30 s,
so a run killed at its deadline still names where its time went. The host
cannot ptrace the container, hence in-process. The replay harness turns it
on per case.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

src/openultrasast/push/cache.py

@@ -244,12 +244,13 @@ class ArtifactCache:
                     elif graph_bytes(payload, budget.deadline_monotonic, target) != sha:
                         raise ValueError("publication_source_changed")
                     target.chmod(0o600)
-                    with target.open("rb") as stream:
-                        os.fsync(stream.fileno())
+                    # No fsync. Durability is not what makes an entry safe to read: `_entry` rejects anything
+                    # whose manifest receipt, payload size or payload SHA-256 does not check, so a write torn
+                    # by a crash is a MISS, never a wrong answer. Two fsyncs per entry were ~18 ms, and a push
+                    # publishes one entry per question -- about 22,000 on a WordPress plugin, minutes of the
+                    # budget spent making a cache survive power loss that it already survives by verification.
                     with (pending / "manifest.json").open("xb") as stream:
                         stream.write(manifest)
-                        stream.flush()
-                        os.fsync(stream.fileno())
                     (pending / "manifest.json").chmod(0o600)
                     if time.monotonic() >= budget.deadline_monotonic:
                         raise TimeoutError("deadline_exhausted")