Add seq4096 fp16 tok coarsen record

[takhir-iota]

Adds a new 10-minute 8xH100 record under records/track_10min_16mb/2026-03-19_Seq4096Fp16TokCoarsen.

Summary:

• canonical exact score: 1.18838751 val_bpb
• canonical total size: 15,937,608 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.

Repro reruns included in the record folder:

• seed=1338: 1.18896209 val_bpb, 15,942,078 bytes
• seed=1339: 1.19055214 val_bpb, 15,942,841 bytes

Files included:

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