feat: add pattern variable binding to C backend

Implements full pattern matching with variable binding for the C backend:

- Extract variable bindings from patterns (Var, Constructor, Tuple, Record)
- Infer C types for bound variables using variant field type tracking
- Handle recursive ADTs with pointer fields and heap allocation
- Dereference pointer bindings automatically for value semantics

Key implementation details:
- variant_to_type: Maps variant names to parent type for tag generation
- variant_field_types: Maps (type, variant) to field types for inference
- Recursive type fields use Type* pointers with malloc/memcpy
- Pattern bindings dereference pointers to maintain value semantics

Examples that now work:
- match opt { Some(x) => x, None => 0 }
- match tree { Leaf(n) => n, Node(l, r) => sum(l) + sum(r) }

Updates documentation to reflect C backend progress.

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

docs/OVERVIEW.md

@@ -181,7 +181,7 @@ fn processAny(x: Int @latest): Int = x  // any version
 
 ### Planned (Not Yet Fully Implemented)
 
-- **Full C Backend**: Basic functions work, closures/lists/pattern variables pending
+- **C Backend Lists**: Closures and pattern matching work, lists pending
 - **Auto-migration Generation**: Migration bodies stored, execution pending
 
 ---
@@ -234,7 +234,7 @@ Quick iteration with type inference and a REPL.
 
 | Limitation | Description |
 |------------|-------------|
-| **Limited C Backend** | Basic functions work, closures/lists pending |
+| **Limited C Backend** | Functions, closures, ADTs work; lists pending |
 | **No Package Manager** | Can't share/publish packages yet |
 | **New Paradigm** | Effects require learning new concepts |
 | **Small Ecosystem** | No community packages yet |
@@ -370,11 +370,12 @@ Values + Effects        C Code → GCC/Clang
 - ✅ Test Effect (native testing framework)
 - ✅ Console.readLine and Console.readInt
 - ✅ C Backend (basic functions, Console.print)
+- ✅ C Backend closures and pattern matching
 - ✅ Watch mode / hot reload
 - ✅ Formatter
 
 **In Progress:**
-1. **C Backend Extensions** - Closures, lists, pattern variable binding
+1. **C Backend Lists** - List operations pending
 2. **Schema Evolution** - Type system integration, auto-migration
 3. **Error Message Quality** - Context lines shown, suggestions partial