fix From Base operation producing wrong results for fractional inputs

The fractional part loop used native `+=` and `Math.pow()` with BigNumber
objects, causing string concatenation instead of arithmetic addition.
Replace with BigNumber `.plus()` and `.pow()` methods.

Closes #2240

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: williballenthin

src/core/operations/FromBase.mjs

@@ -51,9 +51,10 @@ class FromBase extends Operation {
         if (number.length === 1) return result;
 
         // Fractional part
+        const radixBN = new BigNumber(radix);
         for (let i = 0; i < number[1].length; i++) {
             const digit = new BigNumber(number[1][i], radix);
-            result += digit.div(Math.pow(radix, i+1));
+            result = result.plus(digit.div(radixBN.pow(i + 1)));
         }
 
         return result;

tests/operations/index.mjs

@@ -75,6 +75,7 @@ import "./tests/Float.mjs";
 import "./tests/FileTree.mjs";
 import "./tests/FletcherChecksum.mjs";
 import "./tests/Fork.mjs";
+import "./tests/FromBase.mjs";
 import "./tests/FromDecimal.mjs";
 import "./tests/GenerateAllChecksums.mjs";
 import "./tests/GenerateAllHashes.mjs";

tests/operations/tests/FromBase.mjs

@@ -0,0 +1,66 @@
+/**
+ * From Base operation tests.
+ *
+ * @author Willi Ballenthin
+ * @copyright Crown Copyright 2026
+ * @license Apache-2.0
+ */
+import TestRegister from "../../lib/TestRegister.mjs";
+
+TestRegister.addTests([
+    {
+        name: "From Base: binary integer",
+        input: "1010",
+        expectedOutput: "10",
+        recipeConfig: [
+            {
+                op: "From Base",
+                args: [2],
+            },
+        ],
+    },
+    {
+        name: "From Base: binary fraction",
+        input: "10.1",
+        expectedOutput: "2.5",
+        recipeConfig: [
+            {
+                op: "From Base",
+                args: [2],
+            },
+        ],
+    },
+    {
+        name: "From Base: hex fraction",
+        input: "a.8",
+        expectedOutput: "10.5",
+        recipeConfig: [
+            {
+                op: "From Base",
+                args: [16],
+            },
+        ],
+    },
+    {
+        name: "From Base: octal integer",
+        input: "77",
+        expectedOutput: "63",
+        recipeConfig: [
+            {
+                op: "From Base",
+                args: [8],
+            },
+        ],
+    },
+    {
+        name: "From Base: octal fraction",
+        input: "7.4",
+        expectedOutput: "7.5",
+        recipeConfig: [
+            {
+                op: "From Base",
+                args: [8],
+            },
+        ],
+    },
+]);