cmd/bounds: add an assert for the hdr size being 1.

seems to always be 1.  we have a corresponding assert when
building the CFG, so i guess this can't really go wrong.  just to
double check... seems to be valid.
infevent: more informative error messages.
infevent: add test/panic for if we are replacing a BP.

does this happen?  probably means there's a bug, if so.
infevent: fix bug with which bp we pass to the callback.
infevent: don't panic when replacing a BP.

turns out we do this.. doesn't seem to be causing issues.  i think
we're replacing the callback with the same callback.  but there's
no way to tell...
vis2d: tracking states and state transitions better.

also filtering of the allocator based on the caller, helps a bunch!