feat: add an encoder/decoder with a memory bank

[vijk777]

Summary

• Adds zapbench_z0_enc_dec.py training script for sparse z0 bank + linear encoder/decoder
• Phase 1: decoder training (z0_bank + decoder)
• Phase 2: encoder training to match z0_bank targets
• Generates diagnostic plots (L2 distance, t-SNE) for train/val data
• Supports --max_conditions N for fast testing

To run a quick test:

python -m LatentEvolution.zapbench_z0_enc_dec test_fast \
  --decoder_epochs 2 --enc_epochs 2       \
  --max_conditions 1

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