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

scripts/check_doc_numbers.py

@@ -73,7 +73,7 @@ def derived() -> dict[str, object]:
     }
 
     from benchmarks import correctness_suite
-    from benchmarks.tasks_v3 import TASKS_V3
+    from benchmarks.tasks_v4 import TASKS_V4
 
     return {
         "collected_tests": _collected_tests(),
@@ -87,7 +87,7 @@ def derived() -> dict[str, object]:
         "ab_fixed": ab["fixed"],
         "ab_regressed": ab["regressed"],
         "suite_checks": len(correctness_suite.CHECKS),
-        "v3_tasks": len(TASKS_V3),
+        "v4_tasks": len(TASKS_V4),
         "perf_ms": perf,
     }
 
@@ -158,7 +158,7 @@ CHECKS: list[tuple[str, str, str, str]] = [
         "ab_fixed",
     ),
     ("CLAUDE.md", "A/B suite size", r"correctness_suite\.py` \((\d+) checks\)", "ab_total"),
-    ("CLAUDE.md", "v3 task count", r"the current set \((\d+) tasks", "v3_tasks"),
+    ("CLAUDE.md", "v4 task count", r"the current set \((\d+) tasks", "v4_tasks"),
 ]