feat: add checkpoint migration registry

[Sharkyii]

Describe your changes

Old checkpoints fail to load after model refactors because there's no structured way to evolve the state dict schema. This adds a small migration registry (
migrations.py
) that applies ordered, versioned transformations to a checkpoint's state dict on load — replacing the two ad-hoc rename blocks that already existed in on_load_checkpoint.

Schema versioning uses a monotonic integer instead of the package semver, since Hatch's git-tag-derived versioning makes version unreliable in dev installs.

The two existing migrations ( prefix remap + grid_mlp rename) are the first entries in the registry. A standalone python -m neural_lam.migrate_checkpoint script lets you upgrade a checkpoint file on disk rather than migrating in memory on every load.

Dependencies: none beyond what's already in the project.

Issue Link

#48

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• 🐛 Bug fix (non-breaking change that fixes an issue)
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