test(round-builder): the determinism arm builds twice in two processes, not twice in one

Found while killing mutants: an ordering made to depend on hash() survived
test_two_builds_of_the_same_outbox_are_byte_identical untouched. One interpreter has one hash
seed, so the two builds agreed with each other and with nothing else -- the arm could not tell a
deterministic report from a report that is merely consistent within a process. The ordering arm
caught that mutant, so the seam was witnessed; this one was not witnessing it.

The second build now runs as a subprocess with PYTHONHASHSEED=1. Re-measured with the same
mutant: both arms red, where before only one was.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_round_builder_loadbearing.py

@@ -14,6 +14,7 @@ it cannot parse.
 from __future__ import annotations
 
 import json
+import os
 import shutil
 import subprocess
 import sys
@@ -450,10 +451,37 @@ def test_two_builds_of_the_same_outbox_are_byte_identical(tmp_path: Path) -> Non
 
     Row 4 counts content lines IN ORDER, so a report that reshuffles between builds would read as
     an expert's edit. Both the whole-file bytes and the heading sequence are checked, because two
-    files can differ in bytes for a reason that is not order."""
+    files can differ in bytes for a reason that is not order.
+
+    The second build runs in a SUBPROCESS with a different ``PYTHONHASHSEED``, and that is not
+    decoration: measured 19.09, an ordering made to depend on ``hash()`` survived a second
+    in-process build untouched — one interpreter has one hash seed, so the two builds agreed with
+    each other and with nothing else. Two processes is the cheapest way to be a witness rather
+    than a coincidence."""
     source = _outbox(tmp_path)
     first = _build(tmp_path, outbox=source, rounds_dir=tmp_path / "a").round_dir
-    second = _build(tmp_path, outbox=source, rounds_dir=tmp_path / "b").round_dir
+    second = tmp_path / "b" / "0"
+    env = {**os.environ, "PYTHONHASHSEED": "1"}
+    other = subprocess.run(
+        [
+            sys.executable,
+            "-m",
+            "portfolio_optimiser.evals.round_builder",
+            "--outbox",
+            str(source),
+            "--round",
+            "0",
+            "--rounds-dir",
+            str(tmp_path / "b"),
+            "--ran-at",
+            _RAN_AT,
+        ],
+        capture_output=True,
+        text=True,
+        cwd=Path(rb.__file__).resolve().parents[3],
+        env=env,
+    )
+    assert other.returncode == 0, other.stderr
     for name in ("report.md", "outcome.json"):
         assert (first / name).read_bytes() == (second / name).read_bytes(), name
     headings = [