feat: add mcpp.toml for mcpp build support (#2)

Wires mcpplibs.capi.lua into the mcpp build system. The descriptor:
- targets `capi-lua` (lib) at `src/capi/lua.cppm` (override the
  default lib-root convention which would look at `src/lua.cppm`),
- depends on the upstream Lua 5.4.7 C library shipped via
  mcpp-community/mcpp-index (added 2026-05-09).

mcpp 0.0.3's transitive walker propagates lua's headers into our
compile rule, so the existing `extern "C" { #include <lua.h> ... }`
in `src/capi/lua_headers.h` keeps working with no path tweaking.
Verified locally: `mcpp build` is clean against a fresh `target/`.

fix: lua_newstate signature changed in Lua 5.5 (3 args)

Lua 5.5 added a third 'seed' parameter to lua_newstate().
Use version check to pass the extra argument when needed.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Development environment setup (#1)

* feat: implement mcpplibs.capi.lua — C++23 module binding for Lua C API

- Add src/capi/lua.cppm (module interface) and src/capi/lua.cpp (implementation)
- Wrap lua.h, lauxlib.h, lualib.h in mcpplibs::capi::lua namespace
- Types: State, Number, Integer, CFunction, L_Reg, L_Buffer, etc.
- Constants: status codes, type tags, operators, GC options, hook masks
- Functions: 100+ bindings covering state, stack, push/access, tables,
  globals, calls, coroutines, GC, debug, auxiliary library, standard libs
- Use extern C wrapper header to fix GCC C++ modules linkage issue
- 97 comprehensive Google Test cases covering all API categories
- 4 examples: basic, table, function, eval
- Update xmake.lua, CMakeLists.txt, CI workflow, README, architecture docs
- Design docs and task breakdown in docs/pr/
- Remove old templates.cppm placeholder module

Co-authored-by: SPeak <[email protected]>

* docs: add mcpplibs-capi-lua agent skill

- SKILL.md: quick start, naming map, core API cheat sheet, common patterns,
  build config, caveats (callback signatures, extern C, inline limitation)
- reference.md: complete API reference with all 15 types, 50+ constants,
  100+ functions organized by category with C API correspondence
- Update skills README to list both available skills

Co-authored-by: SPeak <[email protected]>

* fix(ci): build library first to avoid C++ modules race condition

GCC C++ modules require std.gcm to exist before dependent targets
compile. Build mcpplibs-capi-lua first (serialized) then build
remaining targets in parallel.

Co-authored-by: SPeak <[email protected]>

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Co-authored-by: Cursor Agent <[email protected]>