feat: add testing framework with reference project integration

- Add test/ directory with test harness and test files
- Add UDT, DB, FB parser tests
- Add PLC JSON and integration tests
- Test against reference project (239 UDTs, 12 DBs, 37 SCL, 13 XML)
- Fix attribute parsing to handle underscores (S7_SetPoint)
- Add ORGANIZATION_BLOCK support to FB parser

AGENTS.md

@@ -18,6 +18,18 @@ tree-sitter parse <file.scl>  # Parse single SCL file
 # Lua Linting
 luacheck src/                 # Lint LSP server code
 luacheck lua/scl/             # Lint plugin modules
+
+# Unit & Integration Tests
+lua test/run_tests.lua       # Run all tests
+lua test/test_udt.lua        # Run UDT parser tests
+lua test/test_db.lua         # Run DB parser tests
+lua test/test_fb.lua         # Run FB parser tests
+lua test/test_plc_json.lua   # Run plc_json tests
+lua test/test_integration.lua # Run integration tests
+
+# Reference Project
+# Tests use files from: ~/dev/siemens/projects/scl_lang_support_lazyvim_ref_project/
+# Override with: SCL_REF_PROJECT=/path/to/ref lua tests/run_tests.lua
 ```
 
 ## Project Structure