Feat/staking precompile coldkey netuid stake info

[fine135]

Description

Adds getStakeInfoForColdkeyAndNetuid(bytes32 coldkey, uint16 netuid) as a view function to StakingPrecompileV2, allowing smart contracts to query the stake positions a coldkey holds on a given subnet. It returns each hotkey the coldkey stakes to together with its alpha stake, as StakeInfo[] ({ bytes32 hotkey; uint256 stake; }); hotkeys with zero stake on that subnet are omitted. The function enumerates StakingHotkeys and reads get_stake_for_hotkey_and_coldkey_on_subnet per hotkey — mirroring the per-subnet slice of the get_stake_info_for_coldkey runtime API — and explicitly records db_read_gas_cost for every storage read so callers are correctly charged when invoked from within a contract.

Also adds the corresponding StakeInfo struct and function to precompiles/src/solidity/stakingV2.sol, unit tests in precompiles/src/staking.rs, and bumps the runtime spec_version.

Related Issue(s)

• N/A

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

N/A

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

Screenshots (if applicable)

N/A

Additional Notes

The function follows the same pattern as the other view functions in StakingPrecompileV2 (e.g. getStake, getAlphaStakedValidators), charging gas per storage read via handle.record_db_reads::<R>(...) — one read for the StakingHotkeys index plus two per hotkey visited — so an oversized index reverts on gas exhaustion rather than doing unbounded work. netuid is a required parameter and only (hotkey, stake) is returned, since returning the fixed netuid would be redundant. The existing get_stake_info_for_coldkey runtime API is intentionally not reused: its StakeInfo fields are private, and it computes fields we don't need (emission, tao_emission, is_registered, ...), so a dedicated storage read is both necessary and cheaper.

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