docs: playground simulation — Phase 1 spec

docs/docs/design/index.md

@@ -32,6 +32,9 @@ Each page carries a status and links the issues / PRs that carry it out.
 - [Playground & tutorial](playground-and-tutorial.md) — an in-browser, real-time
   compile / codegen / runtime demo, and the WASM vs compile-server question
   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.*
 - [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.*

docs/docs/design/playground-and-tutorial.md

@@ -60,6 +60,13 @@ a pure-Rust tree-sitter runtime (`rust-sitter` → `tree-sitter-c2rust`). No
 
 ## The runtime demo
 
+> The concrete, now-buildable plan for this is
+> [Playground simulation — Phase 1](playground-simulation.md): two protocol
+> instances on a canvas, wired over `@comline/runtime`'s in-memory `duplex()`,
+> with a frame inspector. It drives the frozen IR through the real runtime — no
+> codegen or transpile on the path — so a `comline-core` change is not needed
+> beyond one read-only WASM function.
+
 The full runtime is transport + call framing + routing + dispatch. In a browser:
 
 - **serialisation** (`rmp-serde` / `serde_json`), **call framing** (JSON-RPC),