feat: implement subnet EMA reset mechanism

[calm329]

This PR implements a mechanism for subnet owners to reset negative EMA values that prevent their subnet from receiving emissions.

Changes:

• Add MaxEmaResetCost storage item (default: 100 TAO)
• Add reset_subnet_ema extrinsic (call_index: 125)
• Add get_ema_reset_cost helper function
• Add EmaNotInitialized, SubnetEmaNotNegative, NotEnoughBalanceToPayEmaResetCost errors
• Add SubnetEmaReset event with netuid, who, cost, previous_ema
• Add comprehensive tests for the new functionality

The reset cost is calculated as |EMA| × (1/α), capped at MaxEmaResetCost. The burned TAO is recycled to reduce total issuance.

Related Issue(s)

• Closes Subnets become permanently non-functional with concentrated liquidity – add EMA reset mechanism #2228

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

N/A