Closes the gap the reliability testing left open: no real child had ever tripped
the boundary diff. T2 could not do it, and the reason is structural rather than
bad luck — with read_only: true a write is DEFIANCE, and a well-behaved child
refuses, so the detector never runs against a real delta.
Fix: `roots` is now the WATCHED set and `write_allowed` the CHANGEABLE subset. A
violation is then producible by a child that is OBEYING, which is also the
realistic hazard: nobody's agent defiantly rewrites a repo, but plenty of commands
leave artefacts behind.
T4, run to prove it: write task in root A, plus an instruction to verify a module
in root B (watched, NOT writable) with `python3 -m py_compile`. The child obeyed
perfectly — status=ok, typo fixed, module compiled — and still tripped the diff,
because py_compile dropped __pycache__/ into B. Exit 1, violation named, and A's
authorised edit correctly NOT flagged.
It also served as the in-anger test of this morning's --ignored fix: __pycache__/
is gitignored in B, so `git status --porcelain` reported B as CLEAN on the very
same event that `--porcelain --ignored` caught. Pre-fix, T4 would have PASSED.
The fixture test said the same thing; this says it about a real child.
