feat: support Smart Contract Wallets as grantees

contracts/grant_multisig/grant.rs

@@ -0,0 +1,5 @@
+if grantee.is_contract() {
+    require_cross_contract_auth(grantee, caller)?;
+} else {
+    grantee.require_auth(); // normal account
+}

contracts/grant_multisig/mod.rs

@@ -0,0 +1,2 @@
+pub mod grant;
+pub mod multisig;

contracts/grant_multisig/multisig.rs

@@ -0,0 +1,39 @@
+// contracts/grant_multisig/multisig.rs
+
+use std::collections::HashSet;
+
+/// Mock multi-sig / Smart Contract Wallet
+/// Simulates a contract wallet that approves calls if the caller is authorized
+#[derive(Debug)]
+pub struct MockMultisig {
+    authorized: HashSet<String>, // set of addresses authorized to execute
+}
+
+impl MockMultisig {
+    /// Create a new MockMultisig with a list of authorized addresses
+    pub fn new(auth_list: Vec<&str>) -> Self {
+        let authorized: HashSet<String> = auth_list.into_iter().map(|a| a.to_string()).collect();
+        Self { authorized }
+    }
+
+    /// Simulate execution by an external caller
+    /// Returns true if caller is authorized, false otherwise
+    pub fn execute(&self, caller: &str) -> bool {
+        self.authorized.contains(caller)
+    }
+
+    /// Optional helper to add an authorized signer
+    pub fn add_authorized(&mut self, caller: &str) {
+        self.authorized.insert(caller.to_string());
+    }
+
+    /// Optional helper to remove an authorized signer
+    pub fn remove_authorized(&mut self, caller: &str) {
+        self.authorized.remove(caller);
+    }
+
+    /// Check if this is a contract (always true for multi-sig)
+    pub fn is_contract(&self) -> bool {
+        true
+    }
+}