🐛 fix(symbol): index all system tsf files

Do not drop system-library .tsf units when the file stem differs from top-level symbols.

Add test_provider support for --interpreter=... and an interpreter integration test that validates full indexing plus completion+resolve snippet.

lsp-server/test/test_provider/fixtures.cppm

@@ -22,6 +22,22 @@ export namespace lsp::test::provider
         return ExecutablePathStorage();
     }
 
+    inline std::string& InterpreterPathStorage()
+    {
+        static std::string value;
+        return value;
+    }
+
+    inline void SetInterpreterPath(std::string value)
+    {
+        InterpreterPathStorage() = std::move(value);
+    }
+
+    inline const std::string& InterpreterPath()
+    {
+        return InterpreterPathStorage();
+    }
+
     inline std::filesystem::path FixturesRoot()
     {
         return std::filesystem::path(__FILE__).parent_path() / "fixtures";