Add seq4096 sliding-window fp16 tok coarsen record

[takhir-iota]

Adds a new 10-minute 8xH100 sliding-window record under records/track_10min_16mb/2026-03-19_Seq4096SlidingWindowFp16TokCoarsen.

Summary:

• canonical sliding exact score: 1.17675682 val_bpb
• canonical standard exact score: 1.18874323 val_bpb
• canonical total size: 15,943,260 bytes
• training recipe: seq_len=4096, TRAIN_BATCH_TOKENS=393216, tuned Muon schedule
• export recipe: keep tok_emb.weight in fp16 and coarsen only blocks.5.
• eval recipe: EVAL_STRIDE=64, SW_EVAL_BATCH=32

Repro reruns included in the record folder:

• seed=1338: sliding 1.17675183 val_bpb, 15,949,486 bytes
• seed=1339: sliding 1.17910456 val_bpb, 15,950,789 bytes

Files included:

• canonical train.log
• exact train_gpt.py snapshot used for the run
• submission.json
• both rerun logs for reference