fix(tools): okf_adjudicate exits 2 on a malformed plan

A SegmentationError raised by the plan grammar escaped main() as a
traceback and exit 1, while every other malformed-plan case in the same
file already returned 2. Exit codes are the interface a caller scripts
against, and exit 1 with a traceback is the code an unhandled bug
produces -- it says "this command broke" where the truth is "this file
is not a plan".

The refusal itself is unchanged: nothing was written before and nothing
is written now, and the grammar in src/ is untouched. What changes is
one line on stderr naming the error code, and the exit code.

Both branches that can raise are covered: the pre-write parse of a
non-empty plan, and the required-field check reached through the empty
branch.

The old behaviour was pinned by
test_an_entries_value_that_is_not_a_list_is_still_refused, which
asserted that a wrongly-typed `entries` reaches the caller as a raised
SegmentationError and recorded that as a finding rather than fixing it.
That test is rewritten here, in the same commit as the code, to assert
exit 2 plus the code on stderr. A second test pins the one-line stderr
shape on the non-empty branch.

Suite 1072 -> 1073 passed; ruff and mypy --strict clean.

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

tests/test_adjudicate.py

@@ -325,21 +325,57 @@ def test_an_empty_plan_missing_a_required_field_is_still_refused(
     assert not verdict.exists()
 
 
-def test_an_entries_value_that_is_not_a_list_is_still_refused(tmp_path: Path) -> None:
+def test_an_entries_value_that_is_not_a_list_is_still_refused(
+    tmp_path: Path, capsys: pytest.CaptureFixture[str]
+) -> None:
     """Empty is a judgement; the wrong TYPE is a malformed plan, and the two
     must not collapse. The malformed one still meets the unchanged grammar.
 
-    It reaches the caller as a raised SegmentationError rather than as exit 2,
-    which is pre-existing behaviour for every malformed plan and is left alone
-    here rather than repaired inside a change about empty ones. Recorded as a
-    finding, not fixed."""
+    It reaches the caller as EXIT 2, the same code every other malformed plan
+    already got. A grammar refusal used to escape as a traceback and exit 1,
+    which said "this command crashed" where the truth was "this file is not a
+    plan" -- and exit codes are the interface callers script against."""
     plan_path = empty_proposal(tmp_path)
     written = payload(plan_path)
     written["entries"] = "none"
     plan_path.write_text(json.dumps(written), encoding="utf-8", newline="")
 
-    with pytest.raises(SegmentationError) as excinfo:
-        adjudicate_empty(tmp_path, plan_path)
+    code, _ = adjudicate_empty(tmp_path, plan_path)
 
-    assert excinfo.value.code == "segmentation_plan_invalid"
+    assert code == 2
+    assert "segmentation_plan_invalid" in capsys.readouterr().err
+    assert not (tmp_path / "verdict.json").exists()
+
+
+def test_a_malformed_non_empty_plan_exits_two_with_one_stderr_line(
+    tmp_path: Path, capsys: pytest.CaptureFixture[str]
+) -> None:
+    """The same refusal on the other branch: a plan with entries is parsed
+    BEFORE anything is written, and that parse failing is a malformed plan too.
+
+    One line, because a caller reading stderr to tell malformed from missing
+    should not have to parse a traceback to do it."""
+    plan_path = proposal(tmp_path)
+    written = payload(plan_path)
+    written["entries"][0]["span"] = [10, 3]
+    plan_path.write_text(json.dumps(written), encoding="utf-8", newline="")
+    capsys.readouterr()  # the proposer's own report is not what is under test
+
+    code = okf_adjudicate.main(
+        [
+            "--plan",
+            str(plan_path),
+            "--out",
+            str(tmp_path / "verdict.json"),
+            "--adjudicator",
+            ADJUDICATOR,
+            "--adjudicated-at",
+            AT,
+        ]
+    )
+
+    assert code == 2
+    stderr = capsys.readouterr().err
+    assert len(stderr.strip().splitlines()) == 1
+    assert "segmentation_span_invalid" in stderr
     assert not (tmp_path / "verdict.json").exists()