bench: matrix v4 adds pid_roundtrip; correctness suite grows to 29 with three P&ID checks

tasks_v4.py extends v3 with one task the reader can fail on its own: the
example spec is drawn through pid_from_spec's code and read back by the
graph builder, which never sees the spec. The runner, the chart and the
doc-number gate now default to v4; --matrix v3 / v2 reproduce the earlier
sets, and tests/test_benchmark_v3.py labels its full-matrix run "v3"
explicitly now that the default has moved.

The plan's adapter check expected the two unnumbered lines to come back
with no line number; draw_line builds one from the default number_format
(bare sequence when size/service/spec/insulation are empty), so the task
asserts the two authored numbers on the two process lines and sequence-
built XDATA numbers on the other two instead.

The published A/B report still covers 26 checks; Task 21 re-runs it
against v1.5.1, which is what clears the remaining doc-number drift.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

benchmarks/tasks_v4.py

@@ -0,0 +1,38 @@
+"""The v4 task matrix: v3 plus the P&ID round trip (v1.6, track A).
+
+``pid_roundtrip`` draws a five-item P&ID through the same code the tools call
+and reads it back: the edge count, the absence of dangling ends and the
+instrument index are each derived independently of the drawing code. The
+published competitor reports were never asked this question; the chart shows
+``not_run`` for them, not zero.
+"""
+
+from __future__ import annotations
+
+from benchmarks.tasks_v2 import TASKS_V2, TaskSpec
+from benchmarks.tasks_v3 import TASKS_V3
+
+#: Added in v1.6.0. The reader can fail this one on its own.
+NEW_TASKS_V4: tuple[TaskSpec, ...] = (
+    TaskSpec(
+        "pid_roundtrip",
+        "pid",
+        "Draw a P&ID from a spec and read back a matching graph, index and line list",
+        1.0,
+    ),
+)
+
+TASKS_V4: tuple[TaskSpec, ...] = TASKS_V3 + NEW_TASKS_V4
+
+#: Runner-selectable task sets. v2 and v3 stay addressable so an earlier
+#: report can be reproduced exactly rather than only described.
+MATRICES: dict[str, tuple[TaskSpec, ...]] = {"v2": TASKS_V2, "v3": TASKS_V3, "v4": TASKS_V4}
+
+DEFAULT_MATRIX = "v4"
+
+
+def task_by_id(task_id: str, matrix: str = DEFAULT_MATRIX) -> TaskSpec:
+    for task in MATRICES[matrix]:
+        if task.task_id == task_id:
+            return task
+    raise KeyError(f"{task_id} is not in matrix {matrix}")