[that-girl-kate]

Summary

Implements four smart-contract issues for Zaps:

1. contract: CONTRACT: EMIT DETAILED YIELD ACCRUAL EVENTS WITH CALCULATED USER SHARE UPDATES #992 — Emit detailed accrue_yield events when the yield index compounds
2. contract: CONTRACT: ADD SIGNATURE NONCE VERIFICATION TO PREVENT REPLAY IN SOCIAL PAYMENT #996 — Per-user nonce verification for off-chain signed social payments (replay protection)
3. contract: CONTRACT: ENFORCE NAIRA TOKEN DECIMAL PRECISION CHECKS IN TRANSFER AMOUNTS #998 — Enforce Stellar 7-decimal precision and reject zero/negative Naira transfer amounts
4. contract: CONTRACT: IMPLEMENT YIELD VAULT FEE ON WITHDRAWAL MANAGEMENT #999 — Optional 0.1% protocol exit fee on yield vault withdrawals

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#992 — Detailed yield accrual events

Problem: Indexers and UIs needed a clear on-chain signal when the vault interest index compounds, including enough context to reason about depositor share value.

Changes (contracts/contracts/yield_vault/src/lib.rs):

• checkpoint_index publishes
  env.events().publish((Symbol::new(&env, "accrue_yield"),), (old_index, new_index, total_assets))
• Event fires only when the index actually compounds (new_index != old_index), so no-op checkpoints stay quiet
• Payload gives indexers the pre/post index and current total_assets for share-value updates

Acceptance mapping: Soroban accrue_yield event · old_index · new_index · total_assets · uses env.events().publish()

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#996 — Signature nonce verification (social payment)

Problem: Off-chain signed payment authorizations must not be replayable.

Changes (contracts/contracts/social_payment/src/lib.rs):

• Per-user nonces stored as DataKey::Nonce(Address) in persistent storage
• get_nonce(user) returns the current counter (default 0)
• pay_with_signature requires auth.nonce == current_nonce, then increments to current_nonce + 1 on success
• Mismatched / reused / future nonces return Error::NonceAlreadyUsed before signature verification
• Strengthened unit tests covering reuse rejection, future-nonce rejection, and independent per-user counters

Acceptance mapping: Nonce(Address) in storage · increment on successful execution · reject reused nonces

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#998 — Naira token decimal / amount checks

Problem: Transfer amounts must follow Stellar’s 7-decimal standard and never accept zero or negative values.

Changes (contracts/contracts/naira_token/src/lib.rs):

• STELLAR_DECIMALS = 7; initialize asserts decimals == 7
• Shared require_positive_amount helper (assert!(amount > 0)) used by mint, burn, transfer, transfer_from, and burn_from
• Tests covering 7-decimal init plus ignored panic-path coverage for zero/negative transfers (Soroban v20 abort semantics)

Acceptance mapping: Reject zero/negative transfer values · amount > 0 · Stellar 7-decimal precision enforced at init

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#999 — Yield vault exit fee on withdrawal

Problem: Protocol needed an optional 0.1% exit fee on vault withdrawals, routed to a treasury.

Changes (contracts/contracts/yield_vault/src/lib.rs):

• Default exit fee 10 bps (0.1%); treasury defaults to vault owner at initialize
• On withdraw: fee = (assets_out * fee_bps) / 10000, net sent to caller, fee to treasury
• Admin APIs: set_treasury, set_exit_fee_bps (cap 100 bps), plus treasury / exit_fee_bps views
• YieldWithdrawn event now carries the actual fee (was hardcoded 0)
• Fee can be disabled with set_exit_fee_bps(0); emergency exit paths remain fee-free rescue routes
• Tests for fee routing, zero-fee full payout, and balance updates after fee deduction

Acceptance mapping: fee = (amount * fee_bps) / 10000 · fee → treasury · remaining → caller

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Test plan

• cargo +1.85.0 test -p zaps-yield-vault --lib -- test_withdraw_exit_fee test_withdraw_zero_exit test_withdraw_returns_principal test_accrue_yield_emits_accrue
• cargo +1.85.0 test -p zaps-social-payment --lib -- test_signed_payment_rejects_reused_nonce test_signed_payment_rejects_future_nonce test_get_nonce test_signed_payment_different_users
• cargo +1.85.0 test -p zaps-naira-token --lib -- --skip blacklisted (decimals + positive-amount paths)
• Manual: deposit → advance ledgers → accrue_yield → confirm accrue_yield event topics/data
• Manual: withdraw with default 10 bps → confirm treasury receives fee and user receives net
• Manual: pay_with_signature with stale nonce → NonceAlreadyUsed

Closes #992
Closes #996
Closes #998
Closes #999

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[ONEONUORA]

Great job @that-girl-kate