chore: add minimal pom.xml for GitHub dependency graph identification. (#771)

GitHub's dependency graph cannot detect Maven coordinates from
build.gradle.kts. This minimal pom.xml (not used for building)
allows GitHub to identify the package as `network.lightsail:stellar-sdk`
and track dependents.

Author: overcat

pom.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  This pom.xml is NOT used for building. The project uses Gradle (build.gradle.kts).
+  It exists solely so GitHub's dependency graph can identify the published
+  Maven coordinates (network.lightsail:stellar-sdk) and track dependents.
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>network.lightsail</groupId>
+    <artifactId>stellar-sdk</artifactId>
+    <version>2.2.3</version>
+    <packaging>jar</packaging>
+
+    <name>stellar-sdk</name>
+    <description>The Java Stellar SDK library provides APIs to build transactions and connect to Horizon and Soroban-RPC server.</description>
+    <url>https://github.com/lightsail-network/java-stellar-sdk</url>
+</project>