docs: playground simulation — Phase 2 spec (+ Phase 1 status)

docs/docs/design/index.md

@@ -34,7 +34,11 @@ Each page carries a status and links the issues / PRs that carry it out.
   behind it. *Discussion.*
 - [Playground simulation — Phase 1](playground-simulation.md) — wiring two
   protocol instances over `@comline/runtime`'s in-memory transport, with a frame
-  inspector: the buildable plan for the "simulate machines" demo. *Planned.*
+  inspector: the buildable plan for the "simulate machines" demo. *Built (Phase 1).*
+- [Playground simulation — Phase 2](playground-simulation-phase-2.md) — many
+  connections, fault injection, a virtual clock, record & replay, sandboxed
+  user behaviours, and route A (the real generated module in the browser). The
+  staged plan for everything Phase 1 deferred. *Planned.*
 - [Brand assets](brand-assets.md) — `ComlineProject/brand` as the single source
   of truth for the mark and colours, and how consumers vendor a pinned copy.
   *In use.*