pxx 2.5.3

Changed

• Greenfield gate soundness locked with a regression guard. A clone-from-docs probe
  raised a concern that the regression-relative test gate might vacuously pass broken
  greenfield code (a node appeared to reach COMPLETED on a module with a SyntaxError).
  A deterministic reproduce disproves it: run_loop on a suite that fails from round 1
  terminates LOOP_DETECTED / a non-success code, never COMPLETED
  (tests/test_loop.py::test_greenfield_failing_baseline_never_completes) — the apparent
  pass was a confound (lost per-round log + a sandbox whose state did not match the recorded
  outcome), the same failure mode as the earlier "planner fails greenfield" report. The gate
  is sound; a regression guard now locks that in and the roadmap claim is withdrawn. No
  behaviour change — a test + doc correction that hold the gate to its negative-control
  discipline (reproduce cleanly before carding a gap).