fix(callback): scope register's event to local instead of leaking to _G

[Kenshiin13]

Description

lib.callback.register builds an event name and stores it in a variable named event, but the variable is missing the local keyword. In Lua, an assignment without local writes to the global namespace, so every call to lib.callback.register clobbers _G.event in the calling resource:

function lib.callback.register(name, cb)
    event = cbEvent:format(name)        -- writes to _G.event
    lib.setValidCallback(name, true)
    RegisterNetEvent(event, function(...) ... end)
end

Each resource using ox_lib loads its own copy of imports/callback/*.lua into its own Lua state (per resource/init.lua's lazy loader), so the leak is into the calling resource's globals, not ox_lib's. Any user code that uses event as a variable name has it overwritten the moment it calls lib.callback.register.

Reproduction

-- in any resource using ox_lib
event = 'my-resource-event'
print('before:', event)               -- my-resource-event

lib.callback.register('do_thing', function(source)
    return true
end)

print('after:',  event)               -- __ox_cb_do_thing

If the resource later does TriggerEvent(event) expecting 'my-resource-event', it now fires '__ox_cb_do_thing' — a callback channel it didn't intend to invoke. event is a fairly common variable name in event-driven code, so the collision isn't theoretical.

Fix

Add local to both register functions:

 function lib.callback.register(name, cb)
-    event = cbEvent:format(name)
+    local event = cbEvent:format(name)

[unitysync]

LGTM thanks.