feat(settlement): add slippage protection with oracle-based conversion

[Adeyemi-cmd]

Closes #6

PR Description
Summary
Adds slippage protection to the settlement contract's currency conversion flow. When finalize_settlement() converts resource token volume to settlement currency via the oracle exchange rate, the contract now enforces both a protocol-level MAX_SLIPPAGE_BPS bound (default 1%) and an optional user-defined min_expected_amount parameter.
Changes

• contracts/settlement/src/types.rs — New SettlementArgs struct with token_address, volume, recipient, and min_expected_amount: Option. New SettlementResult struct with net_amount, fee_amount, rate_used.
• contracts/settlement/src/constants.rs — Added MAX_SLIPPAGE_BPS: u32 = 100, MAX_VOLUME, MAX_RATE, DECIMAL_DENOMINATOR.
• contracts/settlement/src/rate_application.rs — New module with get_rate() that fetches the current exchange rate from the PriceOracle contract via cross-contract call.
• contracts/settlement/src/conversion.rs — New module with convert_to_settlement_currency() implementing the conversion and slippage check:
• Computes expected_amount = volume * oracle_rate / 1e7 using checked_mul/checked_div
• Computes slippage_bps = (expected - actual) * 10000 / expected
• Rejects with SlippageExceeded if slippage > MAX_SLIPPAGE_BPS
• Rejects if actual_amount < min_expected_amount (if provided)
• Emits SlippageProtectionTriggered event on rejection
• contracts/settlement/src/lib.rs — Added SlippageExceeded error variant, new finalize_settlement() function returning SettlementResult, and oracle cross-contract client.
• contracts/settlement/src/test.rs — 4 new integration tests using a MockOracle:

1. test_zero_slippage_succeeds — no min_expected_amount, succeeds
2. test_slippage_within_tolerance_succeeds — min_expected at 99% of actual
3. test_slippage_exceeds_tolerance_fails — min_expected > actual by 1 unit
4. test_user_min_expected_amount_higher_than_actual_fails — min_expected at 200x

• contracts/Cargo.toml — Added "settlement" to workspace members.
  Testing
  cargo test --package settlement
  All 17 tests pass (13 existing + 4 new).