[Tests] Add test for CompileNativeAssembly per-file skip

Add CompileNativeAssemblySourcesSkipsUnchangedFiles integration test
that verifies on incremental CoreCLR builds, unchanged .ll files are
not recompiled by the CompileNativeAssembly task while changed .ll
files are still correctly compiled.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: davidnguyen-tech

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/IncrementalBuildTest.cs

@@ -1289,6 +1289,38 @@ public void GenerateJavaStubsAndAssembly ([Values] bool isRelease, [Values] Andr
 			}
 		}
 
+		[Test]
+		public void CompileNativeAssemblySourcesSkipsUnchangedFiles ([Values (AndroidRuntime.CoreCLR)] AndroidRuntime runtime)
+		{
+			if (IgnoreUnsupportedConfiguration (runtime, release: false)) {
+				return;
+			}
+
+			var proj = new XamarinAndroidApplicationProject ();
+			proj.SetRuntime (runtime);
+
+			string abi = "arm64-v8a";
+			proj.SetRuntimeIdentifier (abi);
+
+			using (var b = CreateApkBuilder ()) {
+				b.Verbosity = LoggerVerbosity.Detailed;
+				Assert.IsTrue (b.Build (proj), "first build should have succeeded.");
+
+				// Modify MainActivity to trigger recompilation of typemap sources
+				proj.MainActivity = proj.DefaultMainActivity + Environment.NewLine + "// test comment";
+				proj.Touch ("MainActivity.cs");
+				Assert.IsTrue (b.Build (proj), "second build should have succeeded.");
+
+				Assert.IsFalse (b.Output.IsTargetSkipped ("_CompileNativeAssemblySources"), "`_CompileNativeAssemblySources` should *not* be skipped!");
+
+				// At least one .ll file should have been skipped as up to date (e.g., environment.arm64-v8a.ll)
+				Assert.IsTrue (
+					StringAssertEx.ContainsRegex (@"\[LLVM llc\] Skipping.*up to date", b.LastBuildOutput),
+					"Expected at least one .ll file to be skipped as up to date"
+				);
+			}
+		}
+
 		readonly string [] ExpectedAssemblyFiles = new [] {
 			Path.Combine ("android", "environment.@ABI@.o"),
 			Path.Combine ("android", "environment.@ABI@.ll"),