Add design document summarizing codebase architecture

[Copilot]

Adds doc/design.md covering the full architecture of ngx_lua_cpp, derived from the current codebase.

• Architecture overview — ASCII diagrams showing runtime component interaction: LuaJIT VM ↔ shared library ↔ nginx event loop ↔ thread pool
• Component deep-dives — ngx_lua_cpp_t (Lua API surface), ngx_hooker_t (event loop hook + yield/resume mechanism + runtime co_ctx offset discovery), ngx_warp_t (strand config), iris library roles
• Nginx integration — No-header-dependency approach via minimal forward declarations, runtime symbol resolution with dlsym/GetProcAddress, HTTP and stream subsystem detection
• Threading model — Main warp polling, worker thread dispatch via co_await iris_switch, priority task routing
• Build system — CMake setup, LuaJIT discovery, platform support matrix
• Extension guide — How to add new C++ functions exposed to Lua
• Security note — Warning on the debug console's arbitrary code execution endpoint

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