Handle AllowancesStorage when subnet deregistration

[open-junius]

Is your feature request related to a problem? Please describe.

AllowancesStorage is introduced in #2478, but not handled during subnet deregistration.

Describe the solution you'd like

It is a double map, the netuid is part of second key. need to consider how to clean up efficiently.

Describe alternatives you've considered

No response

Additional context

No response

[MkDev11]

@open-junius thanks for opening the issue. can I work on the issue?

[plind-dm]

@open-junius I found RootAlphaDividendsPerSubnet is not cleared too

[plind-dm]

Instead of cleaning up allowances on deregistration, we can lazily invalidate them using a generation counter:

Add SubnetGeneration: Map<netuid, u64>, incremented each time a netuid is registered
Include the generation in the allowance key: (approver, (spender, netuid, generation)) → amount
On transferStakeFrom, check that the stored generation matches the current one — if not, treat as zero
Zero writes on deregistration, one extra read on consumption. Stale entries become inert automatically.

[plind-dm]

Or how about Bounded cleanup(clean a limited number of entries per block across multiple blocks)?