feat: implement ADT RC - pointer fields in algebraic data types

ADT values with pointer fields (like recursive Tree types) now properly
manage memory:

- Assign unique type tags (starting at 100) to each ADT type
- Track which ADTs have pointer fields that need cleanup
- Generate lux_drop_adt() function with per-ADT drop logic
- Allocate ADT pointer fields with lux_rc_alloc instead of malloc
- Track ADT variables with pointer fields in scope
- Emit field cleanup code at scope exit (switch on tag, decref fields)

Test results:
- ADT test: [RC] No leaks: 6 allocs, 6 frees
- List test: [RC] No leaks: 31 allocs, 31 frees
- Closure test: [RC] No leaks: 8 allocs, 8 frees
- All 263 tests pass

Remaining: early returns, complex conditionals.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -5,3 +5,5 @@
 hello
 test_rc
 test_rc2
+test_adt_rc
+test_closure_rc