One switch (REASAMPLER_UNDO_TRACE) gates every line. Traces span open/close reasons, FX writes, UndoBlock depth, timer standoffs, and load-state hooks.
Fixes ctor exception safety, a ValidatePtr2 guard, false SDK-citation and enforcement claims, the span-depth invariant, and vacuous fx_reclaim tests.
The drain refuses a tick inside a foreign block; applyMode wakes the reclaimer before it opens the span.
daniel