Introduce netuid generation for storage cleaning

[evgeny-s]

Description

Iterating an unbounded StorageDoubleMap is unsafe, so instead of clearing state, we make stale state unreachable.

Design

• New storage pallet_subtensor::NetuidGeneration: Map<NetUid, u64>, bumped on every do_register_network.
• AllowancesStorage secondary key changes from (spender, netuid) to (spender, netuid, generation).
• approve, allowance, increaseAllowance, decreaseAllowance, and transferStakeFrom read the current generation from pallet_subtensor and mix it into the key.
• After a netuid is deregistered and re-registered, the generation differs, so entries written under the previous lifetime land at an unreachable key, and all reads through the precompile default to 0 (ValueQuery).
• No iteration, no cleanup hook, no block-time risk.

Related Issue(s)

• Closes #[issue number]

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe):

Breaking Change

If this PR introduces a breaking change, please provide a detailed description of the impact and the migration path for existing applications.

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run ./scripts/fix_rust.sh to ensure my code is formatted and linted correctly
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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Additional Notes

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[open-junius]

Thanks for the solution, I am wondering if we really need it. After we get next generation netuid, the data related old generation is not useful and never can be updated by any transaction. We can remove them all. And I don't think there will be much data for this map in the real production env.