docs: update documentation with RC implementation status

- C_BACKEND.md: Update memory management from "Leaks" to "Scope-based RC",
  update comparison tables with Koka/Rust/Zig/Go
- LANGUAGE_COMPARISON.md: Add status column to gap tables, add RC row
- OVERVIEW.md: Add C backend RC to completed features, update limitations
- REFERENCE_COUNTING.md: Add "Path to Koka/Rust Parity" section with:
  - What we have vs what Koka/Rust have
  - Remaining work for full memory safety (~230 lines)
  - Performance optimizations for Koka parity (~600 lines)
  - Cycle detection strategy

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

docs/OVERVIEW.md

@@ -295,7 +295,7 @@ Quick iteration with type inference and a REPL.
 ### Not a Good Fit (Yet)
 
 - Large production applications (early stage)
-- Performance-critical code (C backend still basic)
+- Performance-critical code (C backend working, but no advanced optimizations)
 - Web frontend development (no JS compilation)
 - Systems programming (no low-level control)
 
@@ -370,12 +370,14 @@ Values + Effects        C Code → GCC/Clang
 - ✅ C Backend (basic functions, Console.print)
 - ✅ C Backend closures and pattern matching
 - ✅ C Backend lists (all 16 operations)
+- ✅ C Backend reference counting (lists, boxed values)
 - ✅ Watch mode / hot reload
 - ✅ Formatter
 
 **In Progress:**
 1. **Schema Evolution** - Type system integration, auto-migration
 2. **Error Message Quality** - Context lines shown, suggestions partial
+3. **Memory Management** - RC working for lists/boxed, closures/ADTs pending
 
 **Planned:**
 4. **SQL Effect** - Database access