margin_pool: clamp decrease_supply_absolute to total_supply

packages/deepbook_margin/sources/margin_pool/margin_state.move

@@ -76,9 +76,12 @@ public(package) fun increase_supply_absolute(self: &mut State, amount: u64) {
 }
 
 /// Decrease the supply given an absolute amount. Used when the supply needs to be
-/// decreased without decreasing shares.
+/// decreased without decreasing shares. Clamped to `total_supply`: any excess
+/// represents bad debt that exceeds the supplier pool's absorptive capacity and
+/// is uncollectible by design (was never backed by vault cash).
 public(package) fun decrease_supply_absolute(self: &mut State, amount: u64) {
-    self.total_supply = self.total_supply - amount;
+    let decrease = amount.min(self.total_supply);
+    self.total_supply = self.total_supply - decrease;
 }
 
 /// Increase the borrow given an amount. Return the individual borrow shares

packages/deepbook_margin/tests/margin_pool/margin_state_tests.move

@@ -82,6 +82,25 @@ fun margin_state_operations_work() {
     test.end();
 }
 
+#[test]
+fun decrease_supply_absolute_clamps_at_zero() {
+    let mut test = begin(test_constants::admin());
+    let clock = clock::create_for_testing(test.ctx());
+    let mut state = margin_state::default(&clock);
+
+    state.increase_supply_absolute(500);
+    assert_eq!(state.total_supply(), 500);
+
+    state.decrease_supply_absolute(600);
+    assert_eq!(state.total_supply(), 0);
+
+    state.decrease_supply_absolute(100);
+    assert_eq!(state.total_supply(), 0);
+
+    destroy(clock);
+    test.end();
+}
+
 #[test]
 fun margin_state_with_supply_and_borrow_accrues_interest() {
     let mut test = begin(test_constants::admin());