feat: implement FBIP (Functional But In-Place) reuse analysis

When rc=1 at update sites, mutate in-place instead of allocating new:

List.reverse:
- Swap element pointers in-place instead of creating new list

List.take:
- Truncate list in-place, decref dropped elements

List.drop:
- Shift elements to front in-place, decref dropped elements

List.map:
- Mutate elements in-place, decref old values before storing new

List.filter:
- Filter in-place by shifting kept elements, decref filtered-out elements

All operations check LUX_RC_HEADER(list)->rc == 1 at runtime and
fall back to allocation when rc > 1 (list is shared).

This completes Phase B performance optimizations:
- B1: Last-use optimization (ownership transfer) ✅
- B2: Reuse analysis (FBIP) ✅
- B3: Drop specialization ✅

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

docs/REFERENCE_COUNTING.md

@@ -33,7 +33,7 @@ The RC system is now functional for lists and boxed values.
 ```
 
 ### What's NOT Yet Implemented
-- Reuse analysis (FBIP) - mutate in-place when rc=1
+- Drop fusion - combining consecutive drops (minor optimization)
 
 ## The Problem
 
@@ -345,7 +345,7 @@ void lux_check_leaks() {
 | Conditionals | Yes | Yes ✅ |
 | Last-use opt | Yes | Yes ✅ (ownership transfer) |
 | Drop special | Yes | Yes ✅ |
-| Reuse (FBIP) | Yes | No |
+| Reuse (FBIP) | Yes | Yes ✅ |
 | Drop fusion | Yes | No |
 
 ---
@@ -438,11 +438,12 @@ Rust's ownership system is fundamentally different:
    - Source variable unregistered from RC tracking
    - No double-free, no unnecessary incref/decref
 
-2. **Reuse analysis (FBIP)** - NOT YET IMPLEMENTED
-   - Detect `rc=1` at update sites
-   - Mutate in-place instead of copy
-   - Major change to list operations
-   - ~300 lines
+2. ~~**Reuse analysis (FBIP)**~~ ✅ DONE
+   - Runtime check `LUX_RC_HEADER(list)->rc == 1`
+   - List.map: mutate elements in-place, decref old values
+   - List.filter: filter in-place, decref removed elements
+   - List.reverse: swap pointers in-place
+   - List.take/drop: truncate/shift in-place
 
 3. ~~**Drop specialization**~~ ✅ DONE
    - Specialized decref functions: `lux_decref_list`, `lux_decref_closure`, etc.
@@ -458,12 +459,13 @@ Rust's ownership system is fundamentally different:
 | A3 | Early returns | ~30 | P1 | ✅ Done |
 | A4 | Conditionals | ~50 | P2 | ✅ Done |
 | B1 | Last-use opt | ~80 | P3 | ✅ Done |
-| B2 | Reuse (FBIP) | ~300 | P3 - Performance | Pending |
+| B2 | Reuse (FBIP) | ~150 | P3 | ✅ Done |
 | B3 | Drop special | ~100 | P3 | ✅ Done |
 
 **Phase A: COMPLETE** ✅ - All leak prevention implemented
-**Phase B: 2/3 DONE** ✅ - Major performance optimizations implemented
-**Remaining: FBIP (~300 lines)** - In-place mutation when rc=1
+**Phase B: COMPLETE** ✅ - All major performance optimizations implemented
+
+Only remaining: Drop fusion (minor optimization, low priority)
 
 ### Cycle Detection