Add checkpoint bundle primitives

[relh]

Summary

• Introduce checkpoint bundle primitives (policy_spec.json + weights.safetensors).
• Add CheckpointPolicy for loading bundle weights and architecture specs.
• Add bundle-focused tests (local bundle write + load).

Changes

• New checkpoint_dir helper for writing/uploading/resolving bundle directories.
• New CheckpointPolicy for safetensors-based loading.
• Tests ensure data_path is relative and bundle URI is file://.

Tests

• not run (stack recut)

[relh]

• Update docs and recipes for checkpoint bundles #4468
• Move MptPolicy compatibility into metta #4467
• Build submission zips from checkpoint bundles #4466
• Adopt checkpoint bundles in training and losses #4465
• Resolve checkpoint bundle URIs and load specs #4464
• Add checkpoint bundle primitives #4463 👈 (View in Graphite)
• main

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