Rewrites the plan_isv_buf comment from an aspirational TODO to a
declarative doc note describing the accepted design gap: backtest
validation intentionally zero-fills plan/ISV state positions [86..92)
because the backtest env kernel does not compute those training-time
introspective signals. The policy treats plan/ISV as advisory
features, so the train/val delta is tolerated in exchange for a lean
backtest env kernel.
Per feedback_no_todo_fixme.md: TODO/FIXME markers are forbidden;
rewrite as declarative production-ready prose or complete the work.
a2624d8b9d