fix(calc): return 0 for zero sell amount in slippage helper

calculate_with_slippage_sell previously fell through to the branch that
returned 1 when amount <= basis_points/10_000, which is true for amount=0
and typical bps values—yielding a bogus minimum of 1. Short-circuit on
zero input so sell quotes stay correct.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/utils/calc/common.rs

@@ -72,6 +72,9 @@ pub const fn calculate_with_slippage_buy(amount: u64, basis_points: u64) -> u64
 /// * basis_points = 500 -> 5% slippage
 #[inline(always)]
 pub const fn calculate_with_slippage_sell(amount: u64, basis_points: u64) -> u64 {
+    if amount == 0 {
+        return 0;
+    }
     if amount <= basis_points / 10000 {
         1
     } else {