Convert warmup/cooldown rates to integers

[grod220]

This pull request implements SIMD-0391, replacing all floating-point (f64) arithmetic in the Stake Program's warmup and cooldown logic with a fixed-point implementation using integer arithmetic. See SIMD for background rationale.

What's new?

• The warmup/cooldown rate calculation now uses BPS values and u128 integer arithmetic.
• All logic related to this now in isolated warmup_cooldown_allowance module
• Delegation struct has been updated to maintain backward compatibility with on-chain data. The warmup_cooldown_rate: f64 field is replaced with _reserved: [u8; 8] (preserves memory layout)

[joncinque]

Looks good overall! Mostly nits and a few suggestions

[joncinque]

This is looking really good! The proptests are excellent, just some bits about the deprecation strategy

[grod220]

Spending a bit of time thinking of this:

// Overflowing u128 is not a realistic scenario except in tests. However, in that case
// it's reasonable to allow activation/deactivation of the account's entire portion.
None => account_portion,

Going to run a few mainnet scenarios. I have a hunch we should saturate the numerators instead.

[joncinque]

This looks ready to me! Feel free to do any other testing you want

[grod220]

@joncinque can you have a look at the latest update? I think saturating the numerators is safer in the extreme cases versus defaulting to the entire account_portion. These cases are not super realistic, but feels more reassuring that there isn't a rate or sizing that could subvert the rate-limit protections. The SIMD would also need updating to reflect this behavior.

[joncinque]

Makes sense to me! Just a nit and a question

[joncinque]

Looks great to me, thanks for addressing all the comments!

[rustopian]

Excellent testing approach 🦾