refactor: collapse repeated argument guards into CheckArgs

[mschmicking]

Found while auditing master for structure. 217 of 673 lines in luastate.cc were argument guards.

The problem

Every method opened with the same preamble — arity check, a type check per argument, then EnsureOpen — each throwing and returning by hand. SetGlobal was typical: 22 lines, of which 2 did the work.

Napi::Value LuaState::SetGlobal(const Napi::CallbackInfo& info) {
	Napi::Env env = info.Env();

	if(info.Length() < 1){
		Napi::TypeError::New(env, "LuaState.SetGlobal Requires 1 Argument").ThrowAsJavaScriptException();
		return env.Undefined();
	}

	if(!info[0].IsString()){
		Napi::TypeError::New(env, "LuaState.SetGlobal Argument 1 Must Be A String").ThrowAsJavaScriptException();
		return env.Undefined();
	}

	if(!EnsureOpen(env)){
		return env.Undefined();
	}

	std::string global_name = info[0].As<Napi::String>().Utf8Value();
	lua_setglobal(lua_, global_name.c_str());

	return env.Undefined();
}

becomes

Napi::Value LuaState::SetGlobal(const Napi::CallbackInfo& info) {
	Napi::Env env = info.Env();
	if(!CheckArgs(info, "SetGlobal", {Arg::String}) || !EnsureOpen(env)){
		return env.Undefined();
	}

	std::string global_name = info[0].As<Napi::String>().Utf8Value();
	lua_setglobal(lua_, global_name.c_str());

	return env.Undefined();
}

What was added

• CheckArgs(info, method, {Arg::String, ...}) — validates arity and types, throws in the established wording.
• ThrowLuaError(env, L, prefix) — builds the message, pops the error, throws. The pop was previously written out at each of the six call sites and is easy to omit, which silently grows the Lua stack. Replaces lua_error_message, whose callers all had to remember to pop.
• EnsureIndexable — the non-table check that was duplicated between SetField and GetField.

Pop and SetTop keep their hand-written checks: their argument is optional, so CheckArgs does not apply. The comments now say so, rather than leaving it looking like an oversight.

Behaviour is unchanged, and that was checked rather than assumed

Messages are now generated, so I verified all fourteen distinct forms are byte-identical to the hardcoded originals — including CollectGarbage's , try nodelua.GC.[TYPE] suffix and the singular/plural split:

"LuaState.DoString Requires 1 Argument"
"LuaState.Call Requires 2 Arguments"
"LuaState.SetField Requires 3 Arguments"
"LuaState.CollectGarbage Argument 1 Must Be A Number, try nodelua.GC.[TYPE]"
"LuaState.RegisterFunction Argument 2 Must Be A Function"
"LuaState.GetField: Value At The Given Index Is Not A Table"
"LuaState Has Already Been Closed"
...

62/62 tests pass, all three examples run.

Result

	Before	After
luastate.cc	672	447
guard boilerplate	217	63
src/ total	899	747

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