non-record: LR warmdown on 1x A40 (1.723 bpb, 8.40MB)

[my-sonicase]

This PR adds a non-record submission under track_10min_16mb.

Summary:
This submission improves over a local MLX baseline (~1.87 bpb) and demonstrates that schedule tuning alone yields significant gains under the 16MB constraint.

• baseline architecture
• no tokenizer or dataset changes
• schedule tuning only:
  • WARMDOWN_ITERS=3600
  • MATRIX_LR=0.06

Result:

• final int8+zlib roundtrip val_bpb: 1.7232
• total submission size int8+zlib: 8,397,395 bytes

Hardware:

• 1x A40
• 600s wallclock-capped run

This is a reproducible non-record submission demonstrating a simple improvement from training schedule tuning under the 16MB constraint.