fix: README.md — complete ablation table with all 13 experiments and CLI flags, add no_sigreg/vicreg_only

fix: SAM 3.1 → jetjodh/sam3.1 (non-gated mirror)

Update README with adapted component stack (DINOv3, Qwen3, SAM3.1, etc.)

Initial MR-JEPA codebase: architecture, training, evaluation, and tests