build Lua from vendored source instead of the prebuilt lua51.lib

The tree carried two prebuilt Lua static libraries and a copy of the Lua
public headers. lua51.lib is 5.1.2 and matched the headers; lua51.vc9.lib
is 5.1.4 and was dead weight — it came second in Ja2_Libraries, so the
linker resolved every Lua symbol out of lua51.lib and never pulled an
object from it. Just as well, since it asks for /DEFAULTLIB:MSVCRT while
we build /MT.

ext/lua-5.1.5 is the upstream tarball unmodified, built as lua51 the way
the other vendored libraries are, and its src directory replaces lua/ as
the home of lua.h, luaconf.h, lauxlib.h and lualib.h. Those four headers
were stock 5.1.2 retabbed, so 5.1.5 is bugfix-only against what the game
compiled against; the bytecode format is unchanged across 5.1.x and every
script under gamedir is plain source anyway. lua/lua.hpp had no includers
and returns to etc/ where upstream keeps it.

/SAFESEH:NO goes with it. Its two stated reasons were lua51.lib and the
smackw32 import library, and both are now gone: every remaining prebuilt
static library is SAFESEH-clean (libexpatMT.lib 5 of 5 members with
@feat.00 = 0x1, RakNetLibStatic.lib 79 of 79), and lld-link emits a
3155-entry SEHandlerTable without it.

The /MT comment blamed the wrong library. lua51.lib carried no linker
directives at all; RakNetLibStatic.lib is what pins us to the static
runtime, with /DEFAULTLIB:LIBCMT and /DEFAULTLIB:libcpmt.

Verified by building all four applications in Debug and Release, and by
linking the tarball's own lua.c against our lua51.lib with the build's
clang-cl flags and running it under Wine: 5.1 stdlib, GC, coroutines and
the x86 __asm fld/fistp lua_number2int fast path all behave. The game
itself could not be launched here — this checkout's gamedir has loose
Data directories but no SLF archives, so VFS aborts on Data\Ambient.slf
long before any script runs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ext/lua-5.1.5/COPYRIGHT

@@ -0,0 +1,34 @@
+Lua License
+-----------
+
+Lua is licensed under the terms of the MIT license reproduced below.
+This means that Lua is free software and can be used for both academic
+and commercial purposes at absolutely no cost.
+
+For details and rationale, see http://www.lua.org/license.html .
+
+===============================================================================
+
+Copyright (C) 1994-2012 Lua.org, PUC-Rio.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.
+
+===============================================================================
+
+(end of COPYRIGHT)