v3.11.2 -- README routing-diagram catch-up to v3.11.1

Patch release. Docs-only. Three more gaps in the ASCII routing
diagram noticed after v3.11.1:

  - /piston/ added to the nginx routes block.
  - execute_code added to mcp_tools in the MCP servers block
    (it was introduced in v3.11.0 but the diagram still only
    showed the original three tools).
  - llama.cpp CPU + CUDA added to the LiteLLM providers block
    (LLAMACPP=1 / LLAMACPP_CUDA=1 serve GGUF + vision-VLM
    weights incl. Surya OCR 2 but were never listed in the
    providers diagram).

No code, config, or behavior changes.