refactor:

[Hendrik-code]

drive every augmentation from a registry instead of if ladders

Which augmentations exist, what they accept, and where they sit in the pipeline were encoded four times over: one dispatch ladder in gpu/transforms.py, two in gpu/transforms_list.py, and one in cpu/transforms.py, totaling ~900 lines across them. They had already drifted -- the list pipelines passed a crop= argument no transform accepts, and ordered SimulateLowRes differently from the sequential one.

Each augmentation class now registers itself. The registry (smauglab/registry.py) is the single source of truth for the class a config key maps to, the parameters it accepts (read from the constructor signature, so there is no second schema to drift), its pipeline order, and its GEO/GE/TA group. smauglab/transforms/build.py dispatches once for all three pipeline modes.

Config handling moves to smauglab/config.py: a sectioned GPU/CPU/pipeline schema, validation that reports all problems in a single file rather than one per run, and "did you mean" suggestions. A flat, section-less config is now rejected -- the two namespaces overlapped enough that GaussianBlurTransform meant different transforms depending on which builder read it.

The three nnU-Net trainers collapse into one. Which sections a config populates decides whether augmentation runs on the dataloader, on the batch, or both, so the CPU/GPU/Hybrid split no longer needs a class each. The class keeps the name nnUNetTrainerDAExtGPU: nnU-Net writes it to every checkpoint and resolves the class from it at inference, so renaming it would leave trained models stranded.

A smauglab CLI answers what exists and whether a config is valid by reading the registry, so it cannot go out of date, and generates the README coverage matrix and the all-augmentations template config that CI checks for staleness.

The .gitignore ignores *.json repo-wide to keep per-experiment configs out (see 7ff2088). That silently swallowed three things this change adds and needs tracked -- the generated template, the test fixtures, and the migrator the config error messages point at -- so each gets an explicit un-ignore.