docs(readme): polish architecture diagram (drop version pins, add LangGraph path, soften wording)

Three surgical edits to the Mermaid diagram added by PR #50 (closes
#3, by @xzlknr). The diagram structure is preserved; only specific
labels and one edge label change.

- Drop hardcoded version numbers from node labels:
  - `ncp-mcp-server v0.1.0` -> `ncp-mcp-server`
  - `ncp-runtime v0.3.6`    -> `ncp-runtime`
  Avoids a doc-bump trap on every runtime/adapter release. Live
  versions are already shown via shields.io badges at the top of
  the README.

- Add the LangGraph adopter path (shipped today as part of Phase
  3A.3):
  `LangGraph (Python via ncp-langgraph) -->|spawns over stdio| Server`
  Both adopter paths now converge at ncp-mcp-server, which matches
  reality: ncp-langgraph's NCPNode.from_subprocess spawns the same
  binary an MCP host would call.

- Soften the Server -> Runtime edge label:
  `dispatches graph` -> `invokes`
  ncp-mcp-server embeds ncp-runtime as a Cargo library dependency,
  not as a separate process. "dispatches graph" implies an IPC
  boundary; "invokes" is a fair compromise for a conceptual diagram
  without going full subgraph.

No changes outside the diagram block. Adjacent prose unchanged.

Signed-off-by: madeinplutofabio <fabio@madeinpluto.com>

Author: madeinplutofabio

README.md

@@ -44,8 +44,9 @@ Instead of “LLM for everything”, you build a graph that:
 
 ```mermaid
 flowchart LR
-  Host["MCP-compatible host"] -->|tools/call| Server["ncp-mcp-server v0.1.0"]
-  Server -->|dispatches graph| Runtime["ncp-runtime v0.3.6"]
+  Host["MCP-compatible host"] -->|tools/call| Server["ncp-mcp-server"]
+  LangGraph["LangGraph (Python via ncp-langgraph)"] -->|spawns over stdio| Server
+  Server -->|invokes| Runtime["ncp-runtime"]
   Manifest["Graph Manifest"] --> Runtime
   Runtime -->|executes| BrickA["Brick (WASM)"]
   Runtime -->|executes| BrickB["Brick (WASM)"]