Prevent savedata corruption and denial-of-service by adding four layers
of protection to the save pipeline:
- Bounded decompression (nullcomp.DecompressWithLimit): caps output size
to prevent OOM from crafted payloads that expand to exhaust memory
- Bounds-checked delta patching (deltacomp.ApplyDataDiffWithLimit):
validates offsets before writing, returns errors for negative offsets,
truncated patches, and oversized output; ApplyDataDiff now returns
original data on error instead of partial corruption
- Size limits on save handlers: rejects compressed payloads >512KB and
decompressed data >1MB before processing; applied to main savedata,
platedata, and platebox diff paths
- Rotating savedata backups: 3 slots per character with 30-minute
interval, snapshots the previous state before overwriting, backed by
new savedata_backups table (migration 0007)
