feat: add Perceus-inspired reference counting infrastructure

Implements Phase 1-3 of the RC system for automatic memory management:

- Add LuxRcHeader with refcount and type tag for all heap objects
- Add lux_rc_alloc, lux_incref, lux_decref, and lux_drop functions
- Update list allocation to use RC (lux_list_new uses lux_rc_alloc)
- List operations (concat, reverse, take, drop) now incref shared elements
- Update boxing functions (box_int, box_bool, box_float) to use RC
- String operations (concat, int_to_string, readLine) return RC strings
- File and HTTP operations return RC-managed strings

The infrastructure is ready for automatic decref insertion at scope exit
(Phase 4) and closure RC (Phase 5) in future work.

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

docs/C_BACKEND.md

@@ -164,27 +164,26 @@ result->length = nums->length;
 
 ## Current Limitations
 
-### 1. Memory Leaks
+### 1. Memory Management (Partial RC)
 
-**Everything allocated is never freed.** This includes:
-- Closure environments
-- ADT values
-- List elements and arrays
-- Strings from concatenation
+RC infrastructure is implemented but not fully integrated:
+- ✅ Lists, boxed values, and strings use RC allocation
+- ✅ List operations properly incref shared elements
+- ⏳ Automatic decref at scope exit (not yet implemented)
+- ⏳ Closures and ADTs still leak
 
-This is acceptable for short-lived programs but not for long-running services.
+**Current state:** Memory is tracked with refcounts, but objects are not automatically freed at scope exit. This is acceptable for short-lived programs but not for long-running services.
 
-### 2. Limited Effects
+### 2. Effects ✅ MOSTLY COMPLETE
 
-Only `Console.print` is supported, hardcoded to `printf`:
+All major effects are now supported:
+- `Console` (print, readLine)
+- `Random` (int, float, bool)
+- `Time` (now, sleep)
+- `File` (read, write, append, exists, delete, isDir, mkdir)
+- `Http` (get, post, put, delete)
 
-```c
-static void lux_console_print(LuxString msg) {
-    printf("%s\n", msg);
-}
-```
-
-Other effects (File, Http, Random, etc.) are not yet implemented in the C backend.
+All effects use evidence passing for O(1) handler lookup.
 
 ### 3. If/Else Side Effects
 
@@ -275,7 +274,7 @@ See [docs/EVIDENCE_PASSING.md](EVIDENCE_PASSING.md) for details.
 
 ## Future Roadmap
 
-### Phase 4: Perceus Reference Counting
+### Phase 4: Perceus Reference Counting 🔄 IN PROGRESS
 
 **Goal:** Deterministic memory management without GC pauses.
 
@@ -284,7 +283,18 @@ Perceus is a compile-time reference counting system that:
 2. Detects when values can be reused in-place (FBIP)
 3. Guarantees no memory leaks without runtime GC
 
-**Example - reuse analysis:**
+**Current Status:**
+- ✅ RC infrastructure (header, alloc, incref/decref, drop)
+- ✅ Lists use RC allocation with proper element incref
+- ✅ Boxed values (Int, Bool, Float) use RC allocation
+- ✅ Dynamic strings use RC allocation
+- ⏳ Automatic decref at scope exit (TODO)
+- ⏳ Closure RC (TODO)
+- ⏳ Last-use optimization (TODO)
+
+See [docs/REFERENCE_COUNTING.md](REFERENCE_COUNTING.md) for details.
+
+**Example - reuse analysis (future):**
 ```lux
 fn increment(xs: List<Int>): List<Int> =
     List.map(xs, fn(x) => x + 1)