Add deterministic percentage rollout to feature flags

[RUKAYAT-CODER]

Background

Flags are boolean-only, preventing gradual rollout. Add deterministic percentage-based bucketing.

Files to modify (2-3)

src/lib/feature-flags/index.ts
src/lib/feature-flags/store.ts

Acceptance Criteria

Implemented across the listed files

Unit/integration tests added or updated and passing

No regression; follows project coding standards

[Marvy54]

@Marvy54 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi team, kindly assign me to this issue, I’ll be happy to contribute

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Marvy54 to this issue.

[drips-wave]

Congratulations, @Marvy54! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @Marvy54: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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