[hmr] `WeakDerived` or something similar 🤔

[CNSeniorious000]

For a standard reaction model:

##########  ---->  ###########  ---->  ##########
# Signal #         # Derived #         # Effect #
##########  < - -  ###########  < - -  ##########

A Derived referenced only by Signals is unreachable and should be GC’d, yet we can’t make every Signal → Derived reference weak or the Derived may vanish too soon.

Solution: give Derived an unsubscribe method that disposes it once no BaseComputation holds it.

Edit: or in a BaseComputation.__del__

We may also need to remind users not to treat Derived as a lazy Effect. But wait, then what can they use if they just need a lazy Effect? So we may implement a weak=False attribute in Derived and AsyncDerived to allow user control the lifespan of a Derived (just like we provide force_flush=True for Batchs).

[linear]

PROM-73 [hmr] `WeakDerived` or something similar