[18] Replace opaque u32 basis-points multiplier enum in stake-vault::get_multiplier #18

Problem Statement

stake-vault/src/lib.rs::get_multiplier returns a raw u32 representing basis points (100=1.0x, 120=1.2x, 200=2.0x). Cross-contract callers (e.g., quest-engine::review_submission) hardcode the meaning: let learner_amount = if calculated_boost > base_learner_amount { base_learner_amount } else { calculated_boost }; — this comparison is meaningless without knowing the divisor (100).

Calling get_multiplier from a new contract without reading the source code in detail can lead to silent bugs in payout scaling.

Why It Matters

• Cross-contract semantics encoded in numeric interpretation is fragile.
• New contributors will be confused why "200" returns for "2.0x."
• Opaque u32 makes integration test assertions brittle.

Expected Outcome

• Define a pub enum MultiplierBps { None = 100, Low = 120, High = 200 } (or similar) and have get_multiplier return it.
• Update QuestEngine to consume the enum and compute payouts accordingly.
• Document the basis-points convention in the rustdoc for get_multiplier.
• Provide a MultiplierBps::as_bps() -> u32 accessor.

Acceptance Criteria

• QuestEngine tests still pass.
• New test: enum round-trip preserves numeric value.
• Readme updated.

Implementation Notes

• Use Soroban's #[contracttype] for the enum so it's serializable if ever stored.
• Survey all call sites before migration.

Files / Modules Affected

• contracts/stake-vault/src/lib.rs
• contracts/stake-vault/src/types.rs
• contracts/quest-engine/src/lib.rs
• contracts/stake-vault/README.md

Dependencies

None.

Difficulty

Medium.

Estimated Effort

3–4 hours.

Suggested Labels

api, quality, P2, stake-vault, quest-engine