TPG.Config.ValidateE2SFSandbox

Check whether the tpg_e2sf_sandbox addon is mounted, and warn if not. The E2/Starfall tpg* chip functions have to exist as a REAL addon under garrysmod/addons/, normally a junction/symlink to this repo's addons/tpg_e2sf_sandbox folder, since git does not carry filesystem links. That is easy to forget on a fresh deploy, so this checks for one of its files and prints a loud warning rather than letting the chip functions silently not exist. Detection is indirect: it does not check for the sandbox addon itself, it checks for autorun/server/tpg_e2sf_stub.lua under the LUA search path as a stand-in for "the junction is in place". Sets TPG.E2SFSandboxMounted for anything else that needs to know. Even when mounted, the chip functions only work while TPG is the active gamemode, not in plain sandbox; see addons/tpg_e2sf_sandbox and README.md.