lif_integrate

The membrane step of the brain module's leaky integrate-and-fire network, one thread per neuron. It drains this step's slot of the fixed-point input ring into the neuron's synaptic conductance, advances the membrane by exponential Euler on dv/dt = (vRest - v + g) / tauM and dg/dt = -g / tauSyn, holds a refractory neuron at reset, draws the neuron's external Poisson event from its stim rate, and on a spike resets it, appends its index to this step's spike list, and bumps the total and its readout group's counter. Every neuron writes its activity trace, normalised membrane and spike flag into lane 0 of the visualisation buffer the neuron cloud draws from. decayMem and decaySyn are exp(-dt / tau) computed once by the module; the step index comes from the GPU clock so several steps can be queued in one frame with no CPU write between them.