Store the legacy `.graphite` document bytes verbatim inside the working copy (dual-write soak). Synchronous (hot-path safe via `write_non_blocking`): called at the autosave boundary alongside the registry snapshot. The bytes are opaque to `Gdd` — it never deserializes them. TODO: Add feature gate for legacy embedding
(&self, bytes: &[u8])
source not stored for this graph (policy: none)
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