Bottom half median owner alpha stake

[JohnReedV]

Summary

This PR updates subnet registration so the initial registration liquidity is split between the new subnet pool and the owner’s starting stake, using the bottom-half median subnet alpha price instead of the full-subnet median.

Previously, registration:

• used the existing median subnet alpha price,
• seeded the new subnet pool with the legacy network_min_lock-style 1:1 initialization,
• granted owner stake based on the earlier registration-alpha flow,
• and recycled the remainder.

With this change, registration now:

• computes alpha from the current lock cost using the median price of the bottom 50% of subnet alpha prices,
• injects 25% of the lock cost as TAO into the new subnet pool,
• injects 50% of the converted alpha into the pool as free alpha liquidity,
• grants the remaining 50% of the converted alpha to the subnet owner as staked alpha,
• and recycles the remaining locked TAO that is not used to seed the pool.

In one line:
Registering a subnet now prices registration alpha off the bottom-half median subnet market price, seeds the pool with 25% TAO + half the converted alpha, and gives the other half of that alpha to the owner as stake.

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