refactor(quest-engine): unify batch and single review payout logic

[Muhammadjazuli]

Summary

Resolves the parity gap between review_submission and batch_review_submissions in the QuestEngine contract.

Previously, batch_review_submissions used a flat payout (fee + base amount) with no staking-multiplier logic. This meant bulk approvals behaved differently from individual approvals, violating the contract's intended invariants.

Changes

contracts/quest-engine/src/lib.rs

• Extracted approve_submission_inner — an internal helper that encapsulates the full approval flow:
  • 15 % platform fee transferred to the reward pool
  • StakeVault multiplier query
  • Boost path (multiplier ≥ 100): pays base from escrow, draws delta from reward pool via distribute_reward
  • Penalty path (multiplier < 100): pays reduced amount from escrow, penalty goes to reward pool
  • Sets submission.status = Approved
• review_submission delegates the approval branch to the helper (no behaviour change)
• batch_review_submissions replaces the old inline flat-payout loop with a call to the helper per learner

contracts/quest-engine/src/test.rs

• Added MockStakeVaultPerLearner: stores per-learner multipliers so mixed-tier batch tests work
• New tests:
  • test_batch_review_with_120_multiplier — boost path verified in batch
  • test_batch_review_with_200_multiplier — larger boost verified in batch
  • test_batch_review_with_80_multiplier — penalty path verified in batch
  • test_batch_review_mixed_multiplier_tiers — three learners each with a different tier (120×, 80×, 100×) in the same batch run
  • test_batch_review_fails_when_pool_cannot_cover_boost — panics when pool has insufficient balance
  • test_batch_review_paused_panics — pause check propagated to batch
  • test_batch_review_already_reviewed_panics — double-approval guard in batch

Acceptance criteria

• Batch approval applies the same fee and multiplier behaviour as single approval
• A change to single-review payout rules does not require duplicating the logic
• Batch review tests cover mixed multiplier tiers and failure cases

Verification

cargo fmt --all -- --check  ✓
cargo clippy --all-targets --all-features -- -D warnings  ✓
cargo test -p quest-engine   61 passed; 0 failed  ✓

closes #88