How do you fix "Class not found" / Composer autoload errors?

An Error: Class "App\Services\Foo" not found almost always means one of: a namespace that doesn’t match the file path (PSR-4), a forgotten composer dump-autoload after adding a class, a service-container binding lost in a merge, or a case-sensitivity mismatch that works on macOS but breaks on a Linux server. We trace the autoload map, fix the root cause, and add a test so the wiring stays intact.