Assemble the ID string probe once, outside the source tree

Each fixture test assembled Documentation/IdStringProbe.il separately,
and ilasm writes its output next to the input: on Windows the second
test failed with a sharing violation (0x80070020) because the first
test's PEFile still had the previous output open. The probe is now
assembled once per test run from a temp copy of the .il, so the tests
share one PEFile and nothing is written into the source tree, which
also makes the per-directory .gitignore unnecessary.

Assisted-by: Claude:claude-fable-5:Claude Code

ICSharpCode.Decompiler.Tests/Documentation/.gitignore

@@ -1,2 +0,0 @@
-/IdStringProbe.dll
-/IdStringProbe.pdb

ICSharpCode.Decompiler.Tests/Documentation/IdStringProviderTests.cs

@@ -2191,11 +2191,30 @@ namespace ModreqParams
 		// and IdStringProbe.xml the unmodified xml doc file MSVC generated for it; every
 		// member key MSVC wrote must be reachable through the ID string candidates.
 
-		static async Task<PEFile> AssembleIdStringProbe()
+		// Assembled once per test run: ilasm writes its output next to the input, so the
+		// .il is copied to a unique temp path first (no output in the source tree), and
+		// the result is shared because re-assembling to the same path would fail on
+		// Windows with a sharing violation while an earlier test's PEFile holds the
+		// previous output open.
+		static readonly Lazy<Task<PEFile>> idStringProbeAssembly = new(async () => {
+			string sourcePath = Path.Combine(Tester.TesterPath, "../../../../Documentation/IdStringProbe.il");
+			string tempPath = Path.Combine(Path.GetTempPath(),
+				"IdStringProbe_" + Guid.NewGuid().ToString("N") + ".il");
+			File.Copy(sourcePath, tempPath);
+			try
+			{
+				string dll = await Tester.AssembleIL(tempPath, AssemblerOptions.Library);
+				return new PEFile(dll);
+			}
+			finally
+			{
+				File.Delete(tempPath);
+			}
+		});
+
+		static Task<PEFile> AssembleIdStringProbe()
 		{
-			string dir = Path.Combine(Tester.TesterPath, "../../../../Documentation");
-			string dll = await Tester.AssembleIL(Path.Combine(dir, "IdStringProbe.il"), AssemblerOptions.Library);
-			return new PEFile(dll);
+			return idStringProbeAssembly.Value;
 		}
 
 		static HashSet<string> CollectIdStringCandidates(PEFile pe)