test: add the e2e package harness

Exercises pkh the way it is used in anger: 63 real source packages
(from hello and micro to dracut, grub2 and the kernel) run through
pull -> chlog --rebuild -> build -> deb, each stage timed, logged and
checked for its artifacts, with the catalog alternating between debian
and ubuntu within every category.

run.py keeps runs self-contained (per-package logs, progressive
results.json, a markdown summary), pulls in parallel while builds run
sequentially to keep pkh's shared chroot/apt/ccache caches warm,
resumes interrupted runs reusing the stages that already succeeded,
and marks known-broken entries as xfail with a reason.

The first catalog run caught four pkh defects within an hour - the
unstable/sid suite alias, the bz2 origs, the hard link extraction and
the wrapper unwrapping - which is exactly what it is for.

tests/e2e/.gitignore

@@ -0,0 +1,2 @@
+# Harness run output: trees, logs, results
+runs/