Add OpenResty guestbook example: Shen web app on nginx+LuaJIT

[pyrex41]

Summary

This PR adds a complete, production-shaped web application example demonstrating how to run Shen code on OpenResty (nginx + LuaJIT). The example is a guestbook API with typed validation rules, a plain HTML+fetch frontend, and can run both under OpenResty and standalone under plain LuaJIT for testing.

Key Changes

• examples/openresty/ — New example directory containing:

  • validate.shen — Typed core with field validation rules, loaded with the typechecker ON. Type errors abort at startup, before the server handles any requests.
  • app.shen — Untyped routing and storage orchestration layer. Dispatches HTTP requests, calls typed validators, and orchestrates storage via Lua interop.
  • app.lua — The Lua glue layer that boots the Shen kernel once per worker (in init_worker_by_lua), marshals JSON ↔ Shen values, and implements the nginx content handler.
  • nginx.conf — OpenResty configuration with two locations: /api/ for the JSON API and / for the static frontend.
  • public/index.html — Plain HTML+fetch frontend (no build step) that demonstrates the ShenScript angle: hand-written JS that could be replaced by Shen compiled to JavaScript.
  • selftest.lua — Standalone test harness that runs the entire app under plain LuaJIT with an in-memory store, exercising typed validation and routing without nginx.
  • json_shim.lua — Minimal JSON codec (encode/decode) used only when cjson is unavailable (off-nginx). OpenResty bundles cjson, so this is never loaded in production.

• Updated examples/README.md — Documents the new example and its purpose.

• Updated root README.md — Adds reference to the OpenResty example.

• .gitignore — Ignores examples/openresty/logs/ (nginx log directory).

Notable Implementation Details

• Boot-once-per-worker pattern: The expensive kernel boot and typechecking of validate.shen happens in init_worker_by_lua, not per-request. This is the critical rule for running shen-lua under nginx.
• Layered design: Typed core (validate.shen under tc +) proves field rules sound at load time. Untyped shell (app.shen under tc -) handles I/O and routing. Both load into the same environment.
• Value marshaling: Request bodies are decoded from JSON and tagged into a val datatype ([s "string"], [n 42], [obj [[k v] ...]]) that Shen patterns match directly. Responses are marshaled back to JSON.
• Storage via Lua interop: The Shen router calls (lua.call "host.store_add" ...) against plain Lua functions backed by an nginx lua_shared_dict, demonstrating non-blocking I/O patterns.
• Testable off-nginx: selftest.lua swaps in an in-memory store and runs the same app.lua dispatch logic under plain LuaJIT, verifying typed validation and routing without nginx.

https://claude.ai/code/session_014PiuPBGphzddJoh5qLjogD