Merge rust-bitcoin/rust-bitcoin#6884: units: Fix div_by_fee_rate_floor precision

### units/src/amount/tests.rs
@@ -360,6 +360,18 @@ fn amount_checked_div_by_fee_rate() {
     assert_eq!(weight, Weight::from_wu(2_100_000_000_000_000_000));
 }
 
+#[test]
+#[cfg(feature = "alloc")]
+fn div_by_fee_rate_floor_preserves_mvb_precision() {
+    // 1001 sat/kvb is 1001 sat per 4,000,000 wu, so a 1000 sat budget funds at most
+    // floor(1000 * 4,000,000 / 1,001,000) = 3996 wu.
+    let weight = sat(1000).div_by_fee_rate_floor(FeeRate::from_sat_per_kvb(1001)).unwrap();
+    assert_eq!(weight, Weight::from_wu(3996));
+
+    // A tiny fee rate over the maximum amount overflows Weight and must error.
+    assert!(Amount::MAX.div_by_fee_rate_floor(FeeRate::from_sat_per_mvb(1)).is_error());
+}
+
 #[test]
 #[cfg(feature = "alloc")]
 fn floating_point() {

### units/src/amount/unsigned.rs
@@ -539,13 +539,19 @@ impl Amount {
     ///
     /// # Errors
     ///
-    /// This can fail only if `fee_rate` is zero, therefore an error returned from this method can
-    /// be treated as infinity.
+    /// Returns an error if `fee_rate` is zero, or if the resulting weight would exceed
+    /// [`Weight::MAX`].
     #[inline]
     pub const fn div_by_fee_rate_floor(self, fee_rate: FeeRate) -> NumOpResult<Weight> {
-        let msats = self.to_msat();
-        match msats.checked_div(fee_rate.to_sat_per_kwu_ceil()) {
-            Some(wu) => R::Valid(Weight::from_wu(wu)),
+        let rate = fee_rate.to_sat_per_mvb() as u128;
+        let sats = self.to_sat() as u128;
+        match (sats * 4_000_000).checked_div(rate) {
+            Some(wu) if wu <= const_casts::u64_to_u128(u64::MAX) =>
+                R::Valid(Weight::from_wu(wu as u64)),
+            Some(_) => R::Error(E::while_doing(MathErrorKind::Overflow {
+                op: MathOp::Div,
+                is_negative: false,
+            })),
             None => R::Error(E::while_doing(MathErrorKind::DivByZero)),
         }
     }