pydiff's CPython arm fails on a relative path, and it looks like a real DIFF

tools/pydiff.py run test/test_nilpy_foo.npy reports a full-file divergence:

DIFF test/test_nilpy_overridden_class_attribute.npy
  exit: cpython=2 pxx=0
  line 1:
    cpython: <no line>
    pxx    : in ctor: derived
  ...

The same file by absolute path is ok (11 lines of stdout agree).

Cause

run_cpython (tools/pydiff.py:57) runs [sys.executable, path] with cwd=os.path.dirname(path). With a relative test/x.npy that is python3 test/x.npy from inside test/ — file not found, CPython exit 2, empty stdout. run_pxx directly below it gets this right: it passes os.path.basename(path) with the same cwd.

One-line fix: os.path.basename(path) in run_cpython too (or make path absolute once at entry, which also covers the bisect/probe subcommands at :147, :440, :470).

Why it matters more than a usage wart

pydiff is the CPython oracle — the tool the debugging playbook says to reach for instead of reasoning. Its failure mode here is not an error message but a plausible-looking DIFF where every pxx line is present and every CPython line is missing, on the exact invocation form (tools/pydiff.py run test/...) an agent working from the repo root naturally types. Read quickly it says "pxx prints, CPython prints nothing", which invites a hunt for a divergence that does not exist. exit: cpython=2 is the only tell.

Found 2026-08-07 while gating [[bug-nilpy-overridden-class-attribute-read-through-an-instance-gives-the-base-value]]; cost one wrong-turn probe before the exit code was noticed.

Gate

tools/testmgr.py --tier full per Track T's rule, plus: the same file diffed by relative and absolute path must give the same verdict.