feat: add Lua 5.1 addon system with .toc loader and /run command

Foundation for WoW-compatible addon support:

- Vendor Lua 5.1.5 source as a static library (extern/lua-5.1.5)
- TocParser: parses .toc files (## directives + file lists)
- LuaEngine: Lua 5.1 VM with sandboxed stdlib (no io/os/debug),
  WoW-compatible print() that outputs to chat, GetTime() stub
- AddonManager: scans Data/interface/AddOns/ for .toc files,
  loads .lua files on world entry, skips LoadOnDemand addons
- /run <code> slash command for inline Lua execution
- HelloWorld test addon that prints to chat on load

Integration: AddonManager initialized after asset manager, addons
loaded once on first world entry, reset on logout. XML frame
parsing is deferred to a future step.

extern/lua-5.1.5/test/bisect.lua

@@ -0,0 +1,27 @@
+-- bisection method for solving non-linear equations
+
+delta=1e-6	-- tolerance
+
+function bisect(f,a,b,fa,fb)
+ local c=(a+b)/2
+ io.write(n," c=",c," a=",a," b=",b,"\n")
+ if c==a or c==b or math.abs(a-b)<delta then return c,b-a end
+ n=n+1
+ local fc=f(c)
+ if fa*fc<0 then return bisect(f,a,c,fa,fc) else return bisect(f,c,b,fc,fb) end
+end
+
+-- find root of f in the inverval [a,b]. needs f(a)*f(b)<0
+function solve(f,a,b)
+ n=0
+ local z,e=bisect(f,a,b,f(a),f(b))
+ io.write(string.format("after %d steps, root is %.17g with error %.1e, f=%.1e\n",n,z,e,f(z)))
+end
+
+-- our function
+function f(x)
+ return x*x*x-x-1
+end
+
+-- find zero in [1,2]
+solve(f,1,2)