Step 3: Additional Info
What actually happened was not a sudden change in the conductivity of the photodiode silicon lattice, but a short-lived buildup of charge as demonstrated by the oscilloscope photographs, akin to the photoelectric effect.
Reverse-bias is *supposed* to improve event capture by expanding the depletion layer, and cause the charge buildup to be shorter lived (but still visible), improving performance... but I could not conspire to get such a thing to work.
If the correct components find me, I will try to repeat this experiment with them in an attempt to cheaply increase sensor area, as well as render the experiment in its entirety more easily reproducible... the current photodiode is a Hamamatsu S1223-01. I have some left over from a failed experiment involving single-photon detection. A very nice product, however rather expensive for a hobbyist.