A Lightning Witness for Schools and Clinics: A $91 Sferics and Electric-Field Warning Node, Its Computed Warning-Time Budget, and the Limits of the 30-30 Rule

Design and reproducible Monte Carlo computation of a $91 two-channel (VLF sferics plus electric-field plate) lightning warning node for schools and clinics. SIMULATED, NOT MEASURED: a single node gives a median 29.2 minutes warning before the first cloud-to-ground strike within 10 km, with zero or negative warning in 12.7% of dangerous storms; a three-node district arrangement ($318) cuts the zero-warning fraction to 5.2%. The all-clear problem and a priced comparison with the 30-30 rule are computed. Includes compute.py and results.json reproducing every number in the paper.