fix(svg): gradient number errors read in the reader's house style

SvgGradients coordinate/radius/stop parsing now goes through one number()
helper that throws '<field> must be a number, got <value>' with the cause
chained, matching the rest of the beta SVG reader instead of leaking the
raw JDK NumberFormatException. Adds a negative test + CHANGELOG entry.

Author: DemchaAV

CHANGELOG.md

@@ -223,6 +223,13 @@ Entries land here as they merge.
   `ListMarker.defaultForDepth(int)` and
   `ListMarker.normalizeItemText(String, boolean)` (`@since 1.8.0`) — and the
   fixed-layout pipeline and the DOCX export both call them.
+- **SVG gradient number errors read in the reader's house style.** A
+  non-numeric gradient coordinate, radius, or stop value (e.g. `x1="abc"`,
+  `r="x%"`, `offset="?"`) leaked the raw JDK `NumberFormatException`
+  ("For input string: …") as the reason. `SvgGradients` now parses through one
+  shared helper that throws `"<field> must be a number, got '…'"` with the
+  cause chained — matching the rest of the beta SVG reader, where the
+  per-element wrapper already names the referencing element.
 
 ### Documentation
 

src/main/java/com/demcha/compose/document/svg/SvgGradients.java

@@ -216,10 +216,10 @@ private static double coordinate(String value, double defaultFraction,
             return userSpace ? defaultFraction * viewportSize : defaultFraction;
         }
         if (v.endsWith("%")) {
-            double fraction = Double.parseDouble(v.substring(0, v.length() - 1)) / 100.0;
+            double fraction = number(v.substring(0, v.length() - 1), "gradient coordinate") / 100.0;
             return userSpace ? fraction * viewportSize : fraction;
         }
-        return Double.parseDouble(v);
+        return number(v, "gradient coordinate");
     }
 
     private static double length(Element gradient, String attr, double defaultFraction,
@@ -231,10 +231,10 @@ private static double length(Element gradient, String attr, double defaultFracti
             return userSpace ? defaultFraction * diagonal : defaultFraction;
         }
         if (v.endsWith("%")) {
-            double fraction = Double.parseDouble(v.substring(0, v.length() - 1)) / 100.0;
+            double fraction = number(v.substring(0, v.length() - 1), "gradient radius") / 100.0;
             return userSpace ? fraction * diagonal : fraction;
         }
-        return Double.parseDouble(v);
+        return number(v, "gradient radius");
     }
 
     /**
@@ -321,9 +321,24 @@ private static double fraction(String value, double defaultValue) {
         }
         String v = value.trim();
         if (v.endsWith("%")) {
-            return Double.parseDouble(v.substring(0, v.length() - 1)) / 100.0;
+            return number(v.substring(0, v.length() - 1), "gradient stop value") / 100.0;
+        }
+        return number(v, "gradient stop value");
+    }
+
+    /**
+     * Parses a gradient numeric value, naming what failed and the offending
+     * input instead of leaking the raw {@link NumberFormatException} message
+     * ("For input string: …"). The cause is chained, and the referencing
+     * element is added by the reader's per-element error wrapper.
+     */
+    private static double number(String value, String what) {
+        try {
+            return Double.parseDouble(value);
+        } catch (NumberFormatException e) {
+            throw new IllegalArgumentException(
+                    what + " must be a number, got '" + value + "'", e);
         }
-        return Double.parseDouble(v);
     }
 
     private static String attrOrStyle(Element element, String property) {

src/test/java/com/demcha/compose/document/svg/SvgIconTest.java

@@ -364,6 +364,25 @@ void gradientCornersWithoutAPdfAnalogueFailLoudly() {
                 .hasMessageContaining("focal");
     }
 
+    @Test
+    void malformedGradientNumberSaysWhatExpectedANumber() {
+        // A non-numeric gradient coordinate must read in the reader's house
+        // style (named + reason) instead of leaking a bare JDK parse message.
+        assertThatThrownBy(() -> SvgIcon.parse("""
+                <svg viewBox="0 0 10 10">
+                  <defs>
+                    <linearGradient id="g" gradientUnits="userSpaceOnUse" x1="abc" y1="0" x2="10" y2="10">
+                      <stop offset="0" stop-color="#000"/><stop offset="1" stop-color="#fff"/>
+                    </linearGradient>
+                  </defs>
+                  <path d="M0 0 H10 V10 Z" fill="url(#g)"/>
+                </svg>
+                """))
+                .isInstanceOf(IllegalArgumentException.class)
+                .hasMessageContaining("must be a number")
+                .hasMessageContaining("abc");
+    }
+
     @Test
     void nodeFormPackagesLayersAtTheRequestedWidth() {
         SvgIcon icon = SvgIcon.parse("""