perf(exp4): trainer memory + visibility — freeze layers, batch 1, step logging, mid-epoch ckpt

fix(exp4): trainer crashed at loss — build mask/class labels explicitly

revert(exp4): labeler back to 648016a baseline — drop patio class + shadow repairs

feat(exp4): patio/deck class + shared taxonomy + frozen review legend

feat(exp4): Phase 2/3 engine — crop exporter, SAM-3 auto-labeler, review page, trainer