Apply PEFT adapters in the Megatron-LM training loop

Parent initiative: #7267

## Work

Add a pre-wrap PEFT hook to the Megatron-LM training path. The hook must apply configured adapters and freeze the base model before distributed model wrapping.

## Done when

- `megatron.training` can apply the migrated PEFT adapters during model setup.
- Adapter application happens before DDP or FSDP wraps the model.
- Base-model parameters are frozen while adapter parameters remain trainable.
- The path supports performant LoRA, canonical LoRA, and DoRA.
- Tests verify adapter placement, parameter freezing, and ordering relative to model wrapping.

Start at the megatron.training model-setup path and trace where DDP or FSDP wrapping occurs. Check how migrated PEFT adapters are configured, then verify adapter placement, base-parameter freezing, trainability, and ordering for performant LoRA, canonical LoRA, and DoRA with tests.

Written by the indexing model from the issue text.