Document full/delta backup + restore

distributed/BACKUPS.md

@@ -0,0 +1,16 @@
+# AGILLM-3.5 distributed master — backup scheme
+
+`distributed/history/` holds **history-kept** backups (never overwritten/squashed,
+so a corrupted upload can't destroy older ones):
+
+- `master_r<round>_<ts>.full.pt`  — complete master (incl optimizer). Self-contained.
+- `master_r<round>_<ts>.delta.pt` — ONLY `core.blocks.*` (the trained layers). Tiny
+  (~1.2 GB vs 6.5 GB). Everything else (emb/ln/ar/sat) is frozen across rounds, so a
+  delta + its base full == the exact model. Each delta records its `base_full`.
+
+Cadence: one backup every ~4 days; every 4th is a full, the rest deltas (no size cap).
+
+## Restore
+Full only:  `restore_from_backup.py <full.pt> master.pt`
+Full+delta: `restore_from_backup.py <base_full.pt> master.pt <delta.pt>`
+Then resume the disagg loop from the restored `master.pt`.