docs(design): multi-file delta-apply pipeline (#1079)

[oferchen]

Summary

• Concise design (under 100 lines) for a receiver-side multi-file delta-apply pipeline that overlaps per-file apply across in-flight files while preserving strict wire-NDX order at acknowledgement and disk-commit boundaries.
• Composes existing primitives: ReceiverDeltaPipeline trait (core: expose NonZeroU8 protocol accessor #1543), bounded work queue, and BoundedReorderBuffer (Ensure workspace protocol metadata matches protocol crate #1407). Threshold gate at PARALLEL_STAT_THRESHOLD = 64 (cross-ref Refactor CLI server handling into dedicated module #1547).
• Risks documented: head-of-line stall (Unify compression level parsing to avoid inconsistent handling #1883) and spill-to-tempfile pending (Ensure release workflow uploads architecture-specific assets #1884). No wire-protocol changes; tcpdump replay remains byte-identical.

Test plan

• Doc renders cleanly in repo file viewer
• Cross-references to core: expose NonZeroU8 protocol accessor #1543, Ensure protocol table validation runs at compile time #1544, Refactor CLI server handling into dedicated module #1547, Ensure workspace protocol metadata matches protocol crate #1407, Unify compression level parsing to avoid inconsistent handling #1883, Ensure release workflow uploads architecture-specific assets #1884 resolve
• No code changes; CI doc/lint jobs pass