FJoinStreamWriter

Order. The head; then one start chunk per open stroke tile with a Flatten start, in the head's order; then one mask frame per brush mask the head lists, in its order; then the load set, which is every byte of the save but the sculpt codes (the header, the stack, the paint, the sculpt index with each tile's summary, and the trailer), so the joiner can load and show the sculpted shapes from the summaries; then the codes, a joiner page at a time: first the open strokes', then those the log after S touches (Prioritise), then by distance from the joiner's interest (ties by page row, then column); then the end frame.

Frames. U8 kind, U32 payload length, the payload, then a U64 H5 (HashBytesV5) over kind, length and payload, so any one changed byte is found. Head: the FJoinHead, each tile with a start flag but not its start (at most kJoinMaxHeadBytes). Start: U32 stroke index in the head, U32 tile index in its stroke, then the tile's start (at most kJoinMaxStartBytes in all). Mask: one BrushMask record's full payload (EncodeBrushMaskRecord), compressed, its hash checked after decompression. Bytes: U64 offset in the save, then at most kJoinFrameBytes of it. End: U64 Bytes frames sent, U64 save bytes sent. The head ends with U32 mask count and, per mask, U64 hash and U32 canonical size, then the limits: U32 MaxRadius_mm, I32 MaxStrength_mm, U32 KeepOutMargin_mm, U8 MaxPaintLayer, U8 flags (bit 0 bPaintGroundClass, 1 bPaintHoles, 2 bStamps, the rest 0), U32 MaxStampRadius_mm, I32 MaxStampHeight_mm, U32 MaxStampTiles.

Editing goes on while the join streams: the plan keeps the moment S alive (copy-on-write), and the joiner gets the records after S on the same ordered link.

Producing. Prepare writes the save into a store (FJoinStreamConfig::MakeSaveStore). Produce frames the stream from the store into a ready ring of about FJoinStreamConfig::ReadAheadBytes, and Read copies out of the ring. The frames are the same bytes whichever store holds the save and whenever Produce runs. Only the code pages' order depends on when Prioritise lands relative to Produce, which the reader allows: frames carry their offsets.

Tier 1. Construct and Prioritise on the game thread; Prepare on any thread (the save's work), at most once; after it, Produce on any thread, one call at a time, and Read on one thread at a time. The ring, under its own lock, is all Produce and Read share, and Produce reads the store outside that lock, so Read never waits on the store. Prioritise may run while Prepare, Produce or Read does: it only queues.