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

README.md

@@ -143,7 +143,7 @@ Self-measurement, produced by scripts in [`benchmarks/`](https://github.com/U-C4
 
 ### Correctness — every release re-proves itself
 
-26 deterministic headless checks against the previous tag and the current tree, each in its own subprocess so a hard crash counts as a miss rather than killing the run.
+29 deterministic headless checks against the previous tag and the current tree, each in its own subprocess so a hard crash counts as a miss rather than killing the run.
 
 | Version | Checks passing | Pass rate | Fixed | Regressed |
 |---|---:|---:|---:|---:|