Modernize copy prevention in utility/signals classes to use = delete (#668)
## Summary
- Replace old-style private-and-undefined copy constructor/assignment in
five `utility/signals` classes with explicit `= delete`
- Affected classes: `SignalHub`, `BufferedSignalHub`,
`PausableSignalHub`, `TokenHub`, `LinkUnit`
- Also explicitly deletes the `LinkUnit` default constructor (was
previously also private-undefined)
- Pure mechanical modernization — no behavioral change
## Motivation
The pre-C++11 idiom of declaring copy ops private with no definition
produces a linker error on misuse rather than a clear compile-time
diagnostic. `= delete` gives an immediate, descriptive compiler error at
the call site and is the modern idiomatic approach.