Replace tinyxml with tinyxml2

[Lpsd]

Summary

Replaces the unmaintained tinyxml library with tinyxml2 11.x (master: https://github.com/leethomason/tinyxml2).

The full XML wrapper layer (CXMLImpl, CXMLFileImpl, CXMLNodeImpl, CXMLAttributeImpl, CXMLAttributesImpl) has been updated to the new API. tinyxml2 ships as a single header and .cpp with no extra dependencies, so it drops straight into vendor/ alongside the existing premake setup.

Motivation

tinyxml(1) has not been updated for a long time.

tinyxml2 is actively maintained by the same original author, handles the same XML subset we use, and has a cleaner modern C++ API.

Notable differences from the old library

This is not a fully transparent swap -- the two libraries have meaningfully different ownership semantics:

• In tinyxml2, the XMLDocument owns all nodes. Deleting a node from inside a wrapper destructor triggers tinyxml2's own recursive child cleanup, which conflicts with our wrapper tree teardown order. The wrapper destructors now leave element cleanup to the document.
• XMLAttribute in tinyxml2 is not a heap-allocatable object -- it is an intrusive member of the element's attribute list. CXMLAttributeImpl was rewritten to hold a const XMLAttribute* pointer and refresh it after any mutation.
• Windows SDK msxml.h declares a global ::XMLDocument class. Using namespace tinyxml2 in CXMLImpl.cpp caused ambiguity at compile time, fixed by using explicit tinyxml2:: prefixes throughout that file.

Test plan

Start the server with default resources and confirm it launches without issues/crashing.

Also worth checking (already tested lightly):

• getResourceInfo() returns correct values (reads from the CopyNode settings node)
• xmlLoadFile / xmlLoadString work from Lua scripts
• XML systems (such as ACL) are working, e.g: aclrequest allow <resource> all successfully saves the acl.xml

Checklist

• Your code should follow the coding guidelines.
• Smaller pull requests are easier to review. If your pull request is beefy, your pull request should be reviewable commit-by-commit.