docs: comprehensive update for sprints 1-9

Update README, ROADMAP, and mdBook to reflect all sprint deliverables:
rich messaging, file transfer, disappearing messages, Go/TypeScript SDKs,
C FFI, mesh networking (identity, store-and-forward, broadcast), and
security hardening. Add 6 new mdBook guides (REPL reference, Go SDK,
TypeScript SDK + browser demo, rich messaging, file transfer, mesh
networking). Check off 16 completed ROADMAP items across phases 3-9.

docs/src/getting-started/demo-walkthrough.md

@@ -324,7 +324,12 @@ Ensure the client has access to the server's TLS certificate. By default, both s
 
 ## Next steps
 
-- [Running the Client](running-the-client.md) -- full CLI reference
+- [REPL Command Reference](repl-reference.md) -- complete list of 40+ slash commands
+- [Rich Messaging](rich-messaging.md) -- reactions, typing indicators, edit/delete
+- [File Transfer](file-transfer.md) -- chunked upload/download with SHA-256 verification
+- [Go SDK](go-sdk.md) -- build Go applications against the qpq server
+- [TypeScript SDK & Browser Demo](typescript-sdk.md) -- WASM crypto in the browser
+- [Mesh Networking](mesh-networking.md) -- P2P, broadcast channels, store-and-forward
 - [MLS (RFC 9420)](../protocol-layers/mls.md) -- how the MLS group operations work
 - [GroupMember Lifecycle](../internals/group-member-lifecycle.md) -- internal state machine details
 - [Delivery Service Internals](../internals/delivery-service.md) -- how the DS queues and delivers messages