Final-diff review measured the worst text the 4 MiB walk budget still
admitted at ~20 s of shlex (cost is linear in admitted bytes, ~7 lex passes
per line, so the line cap was not the binding knob). 1 MiB brings it to
~4 s while a 200-script wrapper graph (~5 KB) and a ~1 MB build script are
still allowed.
lifecycle_scan_root_within_budget: state that it uses a fresh budget,
independent of the full guard's walk (review point on #83821).
e4c1e56d5d