docs: add high-level notes on channel sync

specs/channel-sync-highlevel.md

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+# Channel sync – high-level notes
+
+This document captures a high-level view of how channel-based replication and sync are expected to behave in Malachite.
+
+It is not a formal specification. Instead, it is a starting point for discussions and more detailed specs.
+
+## Goals
+
+- Keep replicas in sync with minimal latency under normal network conditions.
+- Provide clear semantics for how messages are ordered and committed.
+- Make it easy to reason about the behavior of applications built on top of channels.
+
+## Questions to answer
+
+When refining this document into a full spec, we should explicitly answer:
+
+- How do channels behave under sustained network partitions?
+- What are the guarantees about message delivery and reordering?
+- How do we expose progress (e.g. "all messages up to index N are committed") to applications?
+
+As these questions are clarified and implemented, this document can be updated with links to more detailed protocol descriptions and test plans.