feat(stake-vault): replace opaque u32 multiplier with MultiplierBps enum

[malaysiaonelove]

Summary

Closes #18

Replaces the raw u32 basis-points return value from StakeVault::get_multiplier with a typed MultiplierBps enum, eliminating silent payout bugs in cross-contract callers who previously had no way to know the divisor (100) without reading the source.

Changes

stake-vault/src/types.rs

• New pub enum MultiplierBps { None, Low, High } with #[contracttype]
• MultiplierBps::as_bps() -> u32 accessor returning 100 / 120 / 200
• Full rustdoc on the enum, variants, and as_bps()

stake-vault/src/lib.rs

• get_multiplier now returns MultiplierBps instead of u32
• Returns enum variants instead of magic numbers; uses existing TIER_*_STAKE_BOUND constants
• Comprehensive rustdoc with tier table and basis-points usage example
• Deprecated STAKE_TIER_{HIGH,LOW,NONE}_BPS bare constants with #[deprecated] pointing to as_bps()

stake-vault/src/test.rs

• Updated test_get_multiplier to assert MultiplierBps variants
• New test: test_multiplier_bps_round_trip — verifies as_bps() values and scaled-payout arithmetic for all three tiers

quest-engine/Cargo.toml

• Added stake-vault = { path = "../stake-vault" } dependency

quest-engine/src/lib.rs

• StakeVaultInterface::get_multiplier trait now returns MultiplierBps
• Payout arithmetic updated: multiplier as i128 → multiplier.as_bps() as i128

quest-engine/src/test.rs

• Both mock StakeVault contracts updated to return MultiplierBps variants

stake-vault/README.md

• Updated function signature, added MultiplierBps enum table with tier/bps/multiplier columns, as_bps() usage example, and staking tier constants reference

Testing

• All existing quest-engine tests unchanged in behaviour
• New test_multiplier_bps_round_trip enum round-trip test added
• test_get_multiplier updated to compare typed enum variants