refactor: extract IOUAmount exponent bounds check into a helper

The exponent bounds check appeared verbatim in both normalize() and
IOUAmount(Number const&). Extract it into a private
enforceExponentBounds() and call it from both.

Named "enforce" rather than "check" because the function acts on the
value as well as inspecting it: it throws above the range and truncates
to zero below it. That matches the codebase precedent of requireAuth
(a pure predicate) versus enforceMPTokenAuthorization (which mutates).

Defined out of line because a header inline would need STAmount's
offset constants, and STAmount.h already includes IOUAmount.h.

No behavior change: the extracted body is identical to both original
blocks, and the Number constructor still applies it after delegating
construction completes.

include/xrpl/protocol/IOUAmount.h

@@ -40,6 +40,20 @@ private:
     void
     normalize();
 
+    /**
+     * Constrains the exponent to IOUAmount's range, which is narrower than
+     * the one Number normalization enforces.
+     *
+     * The two ends are deliberately asymmetric, matching the class contract:
+     * an exponent above the range is unrepresentable and throws, while one
+     * below it is a silent underflow that truncates the amount to zero.
+     *
+     * @throws std::overflow_error if the exponent exceeds the largest
+     *         representable IOU exponent.
+     */
+    void
+    enforceExponentBounds();
+
     static IOUAmount
     fromNumber(Number const& number);