Neuron registration

[JohnReedV]

Summary

Context: Issue #2351

This PR redesigns neuron registration around a single burn-priced mechanism.

It removes the old PoW + registration window + rate-limit model and replaces it with a continuous market-based flow where registration is always available and price adjusts with demand.

In one line:
No PoW. No windows. No rate limits.
Neuron registration is now always open, and behaves much more like subnet registration.

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What changes

Before	After
PoW-based registration path	Unified burn-based registration path
Registration windows mattered	Registration is continuous
Registration was rate-limited	All registration rate limits are removed
Timing / bot advantage	Price-based admission
Separate PoW vs burned logic	Shared registration flow
No max-price protection	register_limit(...)

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What’s removed

• PoW as the active registration mechanism for non-root neuron registration
• registration windows / interval-based access
• all registration rate limits
• separate PoW and burn registration paths
• tx-extension behavior tied to registration caps / interval fullness
• deprecated PoW / adjustment-era registration state (via migration)

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What’s added

• register_limit(netuid, hotkey, limit_price) for max-price protection
• per-block pricing driven by:
  • BurnHalfLife
  • BurnIncreaseMult
  • BurnLastHalvingBlock
• EVM support for registerLimit(uint16,bytes32,uint64)
• migration to initialize the new pricing model and clear deprecated state

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Important behavior change

register(...) still exists, but it is no longer a PoW registration API.

It now routes into the same burn-based flow as burned_register(...), and its old PoW-shaped fields are effectively legacy compatibility inputs.

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Scope note

This change applies to non-root neuron registration.

root_register(...) remains a separate path and is not part of this redesign.

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