feat: implement closure RC - environments are now memory-managed

Closures and their environments are now properly reference-counted:

- Allocate closures with lux_rc_alloc(sizeof(LuxClosure), LUX_TAG_CLOSURE)
- Allocate environments with lux_rc_alloc(sizeof(LuxEnv_N), LUX_TAG_ENV)
- Enable Lambda in expr_creates_rc_value() to track closure variables
- Add lux_decref() after List higher-order operations (map, filter, fold,
  find, any, all) to clean up inline lambdas

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

Remaining for full memory safety: ADT RC, early returns, conditionals.

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

docs/REFERENCE_COUNTING.md

@@ -15,6 +15,8 @@ The RC system is now functional for lists and boxed values.
 - Polymorphic drop function (`lux_drop`)
 - Lists, boxed values, strings use RC allocation
 - List operations incref shared elements
+- **Closures and environments** - RC-managed with automatic cleanup
+- **Inline lambda cleanup** - temporary closures freed after use
 - **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
@@ -27,7 +29,6 @@ The RC system is now functional for lists and boxed values.
 ### What's NOT Yet Implemented
 - Early return handling (decref before return in nested scopes)
 - Conditional branch handling (complex if/else patterns)
-- Closure RC (environments still leak)
 - ADT RC
 
 ## The Problem
@@ -403,10 +404,10 @@ Rust's ownership system is fundamentally different:
 
 #### Phase A: Complete Coverage (Prevent All Leaks)
 
-1. **Closure RC** - Environments should be RC-managed
-   - Allocate env with `lux_rc_alloc`
-   - Drop env when closure is dropped
-   - ~50 lines in `emit_lambda`
+1. ~~**Closure RC**~~ ✅ DONE - Environments are now RC-managed
+   - Closures allocated with `lux_rc_alloc(sizeof(LuxClosure), LUX_TAG_CLOSURE)`
+   - Environments allocated with `lux_rc_alloc(sizeof(LuxEnv_N), LUX_TAG_ENV)`
+   - Inline lambdas freed after use in List operations
 
 2. **ADT RC** - Algebraic data types with heap fields
    - Track which variants contain RC fields
@@ -444,17 +445,17 @@ Rust's ownership system is fundamentally different:
 
 ### Estimated Effort
 
-| Phase | Description | Lines | Priority |
-|-------|-------------|-------|----------|
-| A1 | Closure RC | ~50 | P0 - Closures leak |
-| A2 | ADT RC | ~100 | P1 - ADTs leak |
-| A3 | Early returns | ~30 | P1 - Edge cases |
-| A4 | Conditionals | ~50 | P2 - Uncommon |
-| B1 | Last-use opt | ~200 | P3 - Performance |
-| B2 | Reuse (FBIP) | ~300 | P3 - Performance |
-| B3 | Drop special | ~100 | P3 - Performance |
+| Phase | Description | Lines | Priority | Status |
+|-------|-------------|-------|----------|--------|
+| A1 | Closure RC | ~50 | P0 | ✅ Done |
+| A2 | ADT RC | ~100 | P1 - ADTs leak | Pending |
+| A3 | Early returns | ~30 | P1 - Edge cases | Pending |
+| 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 total: ~230 lines** - Gets us to "no leaks"
+**Phase A remaining: ~180 lines** - Gets us to "no leaks"
 **Phase B total: ~600 lines** - Gets us to Koka-level performance
 
 ### Cycle Detection