feat: add SavedVariables persistence for Lua addons

Addons can now persist data across sessions using the standard WoW
SavedVariables pattern:

1. Declare in .toc: ## SavedVariables: MyAddonDB
2. Use the global in Lua: MyAddonDB = MyAddonDB or {default = true}
3. Data is automatically saved on logout and restored on next login

Implementation:
- TocFile::getSavedVariables() parses comma-separated variable names
- LuaEngine::loadSavedVariables() executes saved .lua file to restore globals
- LuaEngine::saveSavedVariables() serializes Lua tables/values to valid Lua
- Serializer handles tables (nested), strings, numbers, booleans, nil
- Save triggered on PLAYER_LEAVING_WORLD and AddonManager::shutdown()
- Files stored as <AddonDir>/<AddonName>.lua.saved

Updated HelloWorld addon to track login count across sessions.

Data/interface/AddOns/HelloWorld/HelloWorld.lua

@@ -1,5 +1,11 @@
 -- HelloWorld addon — demonstrates the WoWee addon system
 
+-- Initialize saved variables (persisted across sessions)
+if not HelloWorldDB then
+    HelloWorldDB = { loginCount = 0 }
+end
+HelloWorldDB.loginCount = (HelloWorldDB.loginCount or 0) + 1
+
 -- Create a frame and register for events (standard WoW addon pattern)
 local f = CreateFrame("Frame", "HelloWorldFrame")
 f:RegisterEvent("PLAYER_ENTERING_WORLD")
@@ -10,6 +16,7 @@ f:SetScript("OnEvent", function(self, event, ...)
         local name = UnitName("player")
         local level = UnitLevel("player")
         print("|cff00ff00[HelloWorld]|r Welcome, " .. name .. "! (Level " .. level .. ")")
+        print("|cff00ff00[HelloWorld]|r Login count: " .. HelloWorldDB.loginCount)
     elseif event == "CHAT_MSG_SAY" then
         local msg, sender = ...
         if msg and sender then
@@ -23,6 +30,7 @@ SLASH_HELLOWORLD1 = "/hello"
 SLASH_HELLOWORLD2 = "/hw"
 SlashCmdList["HELLOWORLD"] = function(args)
     print("|cff00ff00[HelloWorld]|r Hello! " .. (args ~= "" and args or "Type /hello <message>"))
+    print("|cff00ff00[HelloWorld]|r Sessions: " .. HelloWorldDB.loginCount)
 end
 
-print("|cff00ff00[HelloWorld]|r Addon loaded. Type /hello to test slash commands.")
+print("|cff00ff00[HelloWorld]|r Addon loaded. Type /hello to test.")

Data/interface/AddOns/HelloWorld/HelloWorld.toc

@@ -1,4 +1,5 @@
 ## Interface: 30300
 ## Title: Hello World
 ## Notes: Test addon for the WoWee addon system
+## SavedVariables: HelloWorldDB
 HelloWorld.lua