Equitable allocation of indivisible goods to agents in online settings is an algorithmic primitive with applications for load balancing, network routing, online marketplaces, and multi-agent systems. We consider a general setting in which allocations are constrained to be bases of discrete polymatroids that arrive online. Our work demonstrates that a simple, myopic algorithm called Brick-Laying, which greedily minimizes the sum of squared loads on agents, achieves a universal and objective-free notion of optimality called majorization minimax-optimality [BDK26] for this setting. As a consequence, Brick-Laying simultaneously guarantees minimax optimal competitive ratios and regret for all Schur-concave and Schur-convex objectives, and for any number of agents and resources (despite being agnostic to problem scale). Departing from popular primal-dual analysis, we employ majorization to compare allocations. We leverage the conjugates of integer partitions -- which act as a discrete dual to majorization -- to characterize worst-case instances for the Brick-Laying algorithm. Our approach reveals a novel structural connection between the geometry of partitions and online equitable allocation.