LR-PRE-AMOUNT: presets with the Grain group apply at any amount

scale_patch interpolated every JSON number as a float, including the
integer grain seed; the fractional seed failed to deserialize, merged()
failed and apply_partial returned the photo unchanged, so a preset with
the Grain group (in the default copy groups) did nothing at any amount
other than 100 %. Integer fields are now discrete like strings and
booleans; float settings and integer literals in preset files still
interpolate.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/develop/src/presets.rs

@@ -162,7 +162,9 @@ fn scale_patch(base: &Value, patch: &Value, t: f64) -> Value {
             }
             Value::Object(out)
         }
-        (Value::Number(b), Value::Number(p)) => {
+        // Integer fields (the grain seed, ids) are discrete like strings: an interpolated seed would
+        // not deserialize as an integer, and the whole preset would silently not apply.
+        (Value::Number(b), Value::Number(p)) if b.is_f64() || p.is_f64() => {
             let (b, p) = (b.as_f64().unwrap_or(0.0), p.as_f64().unwrap_or(0.0));
             serde_json::Number::from_f64(b + (p - b) * t).map(Value::Number).unwrap_or(Value::Null)
         }
@@ -267,6 +269,25 @@ mod tests {
         assert_eq!(lots.light.exposure, 5.0);
     }
 
+    /// A preset with the Grain group (its seed is an integer) used to do nothing at any amount
+    /// other than 100 %: the interpolated seed failed to deserialize and the preset was dropped.
+    #[test]
+    fn amount_keeps_integer_fields_whole() {
+        let mut a = DevelopSettings::default();
+        a.light.exposure = 1.0;
+        a.grain.amount = 40.0;
+        a.grain.seed = 12345;
+        let p = Preset::from_settings("p", "P", "User", &a, &SettingsGroup::default_copy());
+        assert!(p.settings.pointer("/grain/seed").is_some_and(Value::is_u64), "the default copy groups include the grain seed");
+        let base = DevelopSettings::default();
+        for (amount, seed) in [(0.3, 0), (0.5, 12345), (1.5, 12345), (2.0, 12345)] {
+            let out = p.apply(&base, amount);
+            assert!((out.light.exposure - amount.min(5.0)).abs() < 1e-9, "{amount}: exposure {}", out.light.exposure);
+            assert!((out.grain.amount - (40.0 * amount).min(100.0)).abs() < 1e-9, "{amount}: grain {}", out.grain.amount);
+            assert_eq!(out.grain.seed, seed, "{amount}: seed");
+        }
+    }
+
     #[test]
     fn deep_merge_objects() {
         let mut a = json!({"x": {"a": 1, "b": 2}, "y": [1]});