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

lua/scl/db_parser.lua

@@ -104,10 +104,10 @@ local function parse_attributes(attr_str)
   end
 
   local attrs = {}
-  for key, value in attr_str:gmatch("(%w+)%s*:=%s*'([^']+)'") do
+  for key, value in attr_str:gmatch("([%w_]+)%s*:=%s*'([^']+)'") do
     attrs[key] = value
   end
-  for key, value in attr_str:gmatch("(%w+)%s*:=%s*(%w+)") do
+  for key, value in attr_str:gmatch("([%w_]+)%s*:=%s*(%w+)") do
     attrs[key] = value
   end
   return attrs