feat: implement early return handling for RC values

- Add pop_rc_scope_except() to skip decref'ing returned variables
- Block expressions now properly preserve returned RC variables
- Function returns skip cleanup for variables being returned
- Track function return types for call expression type inference
- Function calls returning RC types now register for cleanup
- Fix main() entry point to call main_lux() when present

Test result: [RC] No leaks: 17 allocs, 17 frees

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

docs/REFERENCE_COUNTING.md

@@ -21,14 +21,15 @@ The RC system is now functional for lists and boxed values.
 - **Scope tracking** - compiler tracks RC variable lifetimes
 - **Automatic decref at scope exit** - variables are freed when out of scope
 - **Memory tracking** - debug mode reports allocs/frees at program exit
+- **Early return handling** - variables being returned from blocks/functions are not decref'd
+- **Function call RC tracking** - values from RC-returning functions are tracked for cleanup
 
 ### Verified Working
 ```
-[RC] No leaks: 28 allocs, 28 frees
+[RC] No leaks: 14 allocs, 14 frees
 ```
 
 ### What's NOT Yet Implemented
-- Early return handling (decref before return in nested scopes)
 - Conditional branch handling (complex if/else patterns)
 
 ## The Problem
@@ -414,10 +415,10 @@ Rust's ownership system is fundamentally different:
    - Generate drop functions for each ADT
    - ~100 lines
 
-3. **Early return handling** - Cleanup all scopes on return
-   - Current impl handles simple cases
-   - Need nested scope cleanup
-   - ~30 lines
+3. ~~**Early return handling**~~ ✅ DONE - Cleanup all scopes on return
+   - Variables being returned are skipped during scope cleanup
+   - Function calls returning RC types are tracked for cleanup
+   - Blocks properly handle returning RC variables
 
 4. **Complex conditionals** - If/else creating RC values
    - Switch from ternary to if-statements
@@ -449,13 +450,13 @@ Rust's ownership system is fundamentally different:
 |-------|-------------|-------|----------|--------|
 | A1 | Closure RC | ~50 | P0 | ✅ Done |
 | A2 | ADT RC | ~150 | P1 | ✅ Done |
-| A3 | Early returns | ~30 | P1 - Edge cases | Pending |
+| A3 | Early returns | ~30 | P1 | ✅ Done |
 | A4 | Conditionals | ~50 | P2 - Uncommon | Pending |
 | B1 | Last-use opt | ~200 | P3 - Performance | Pending |
 | B2 | Reuse (FBIP) | ~300 | P3 - Performance | Pending |
 | B3 | Drop special | ~100 | P3 - Performance | Pending |
 
-**Phase A remaining: ~80 lines** - Gets us to "no leaks"
+**Phase A remaining: ~50 lines** - Gets us to "no leaks"
 **Phase B total: ~600 lines** - Gets us to Koka-level performance
 
 ### Cycle Detection