[FEATURE] High-resolution fractional bucketing in flagd provider

[toddbaert]

Sub-issue of open-feature/flagd#1903. See the parent issue and ADR for full details.

Replace the current 100-bucket float-based fractional bucketing with integer arithmetic using MaxInt32 (2,147,483,647) as the max weight sum:

bucket = (uint64(hashValue) * uint64(totalWeight)) >> 32

Weights must be integers and sum to at most MaxInt32. C#/.NET: use uint hash, (ulong) for multiply, >> 32 for shift.

This is a behavioral breaking change (bucket assignments will shift), but the API surface is unchanged.

⚠️ Dependency: complete the nested fractional evaluation feature first: #607

@toddbaert will coordinate the release across providers.