feat(#1375): Scaffold OpNonce storage and batch validation logic

contracts/loan_manager/src/nonce.rs

@@ -0,0 +1,47 @@
+use soroban_sdk::{contracterror, contracttype, Address, Env};
+
+#[contracttype]
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub enum OpKind {
+    ProcessDefaults,
+    ApproveLoans,
+    GovernanceExecute,
+}
+
+#[contracttype]
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct OpNonceKey {
+    pub address: Address,
+    pub op_kind: OpKind,
+}
+
+#[contracterror]
+#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
+#[repr(u32)]
+pub enum BatchError {
+    NonceReused = 1,
+    BatchWindowExpired = 2,
+}
+
+pub fn consume_nonce(env: &Env, address: Address, op_kind: OpKind, supplied_nonce: u64) -> Result<(), BatchError> {
+    let key = OpNonceKey { address, op_kind };
+    let current_nonce: u64 = env.storage().persistent().get(&key).unwrap_or(0);
+
+    if supplied_nonce != current_nonce + 1 {
+        return Err(BatchError::NonceReused);
+    }
+
+    env.storage().persistent().set(&key, &supplied_nonce);
+
+    // Bump TTL for the entry
+    env.storage().persistent().extend_ttl(&key, 1000, 100000);
+
+    Ok(())
+}
+
+pub fn validate_batch_window(env: &Env, valid_until_ledger: u32) -> Result<(), BatchError> {
+    if env.ledger().sequence() > valid_until_ledger {
+        return Err(BatchError::BatchWindowExpired);
+    }
+    Ok(())
+}