tao: add subnet alpha token transfers and balance queries

[conorpp]

Summary

• Add support for Bittensor Dynamic TAO (dTAO) subnet alpha tokens
• Alpha balances are queried by enumerating all hotkeys via StakingHotkeys storage and summing Alpha balances per subnet
• Transfers use UTXO-style position selection across hotkey positions, automatically batched with Utility.batch_all when spanning multiple hotkeys
• Contract address is simply the subnet netuid (e.g. "64" for subnet 64) — hotkeys are resolved dynamically
• Adds SubtensorModule.StakeTransferred event parsing for transfer_stake results

Test plan

• Unit tests for ParseAlphaContract parsing (valid netuids, edge cases, errors)
• Unit tests for alpha builder: single position, multi-position batch, exact amount, insufficient balance, no positions, invalid contract, non-TAO rejection
• Integration test: query alpha balance for a known coldkey/subnet
• Integration test: build and submit a transfer_stake transaction
• Verify event parsing for StakeTransferred events

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