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/sort.lua

@@ -0,0 +1,66 @@
+-- two implementations of a sort function
+-- this is an example only. Lua has now a built-in function "sort"
+
+-- extracted from Programming Pearls, page 110
+function qsort(x,l,u,f)
+ if l<u then
+  local m=math.random(u-(l-1))+l-1	-- choose a random pivot in range l..u
+  x[l],x[m]=x[m],x[l]			-- swap pivot to first position
+  local t=x[l]				-- pivot value
+  m=l
+  local i=l+1
+  while i<=u do
+    -- invariant: x[l+1..m] < t <= x[m+1..i-1]
+    if f(x[i],t) then
+      m=m+1
+      x[m],x[i]=x[i],x[m]		-- swap x[i] and x[m]
+    end
+    i=i+1
+  end
+  x[l],x[m]=x[m],x[l]			-- swap pivot to a valid place
+  -- x[l+1..m-1] < x[m] <= x[m+1..u]
+  qsort(x,l,m-1,f)
+  qsort(x,m+1,u,f)
+ end
+end
+
+function selectionsort(x,n,f)
+ local i=1
+ while i<=n do
+  local m,j=i,i+1
+  while j<=n do
+   if f(x[j],x[m]) then m=j end
+   j=j+1
+  end
+ x[i],x[m]=x[m],x[i]			-- swap x[i] and x[m]
+ i=i+1
+ end
+end
+
+function show(m,x)
+ io.write(m,"\n\t")
+ local i=1
+ while x[i] do
+  io.write(x[i])
+  i=i+1
+  if x[i] then io.write(",") end
+ end
+ io.write("\n")
+end
+
+function testsorts(x)
+ local n=1
+ while x[n] do n=n+1 end; n=n-1		-- count elements
+ show("original",x)
+ qsort(x,1,n,function (x,y) return x<y end)
+ show("after quicksort",x)
+ selectionsort(x,n,function (x,y) return x>y end)
+ show("after reverse selection sort",x)
+ qsort(x,1,n,function (x,y) return x<y end)
+ show("after quicksort again",x)
+end
+
+-- array to be sorted
+x={"Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"}
+
+testsorts(x)