Dataset503_MicroridgeMembraneFirst. Same 477 tiles, same architecture,
same 100 epochs; only which class wins a contested label pixel changed.
v2 trained on labels where microridges had erased 69.6% of the membrane,
so it predicted a broken membrane and called membrane pixels microridge.
Frozen test: cell_membrane Dice 0.471 -> 0.707, boundary F1 0.659 -> 0.874,
microridge Dice 0.877 -> 0.852.
Add v2 microridge model: weights, plans, and provenance record
