✨ Python LLVM backend implementation (experimental)

- Created llvmlite-based LLVM backend in src/llvm_py/
- Implemented all MIR14 instructions (const, binop, jump, branch, ret, compare, phi, call, boxcall, externcall, typeop, newbox, safepoint, barrier)
- Experimental LoopForm support
- ~2000 lines of clean Python code vs complex Rust/inkwell
- Useful for PHI/SSA validation and rapid prototyping
- Added documentation to CLAUDE.md

This was created while waiting for ChatGPT's investigation of BuilderCursor issues.

src/llvm_py/instructions/newbox.py

@@ -0,0 +1,116 @@
+"""
+NewBox instruction lowering
+Handles box creation (new StringBox(), new IntegerBox(), etc.)
+"""
+
+import llvmlite.ir as ir
+from typing import Dict, List, Optional
+
+def lower_newbox(
+    builder: ir.IRBuilder,
+    module: ir.Module,
+    box_type: str,
+    args: List[int],
+    dst_vid: int,
+    vmap: Dict[int, ir.Value],
+    resolver=None
+) -> None:
+    """
+    Lower MIR NewBox instruction
+    
+    Creates a new box instance and returns its handle.
+    
+    Args:
+        builder: Current LLVM IR builder
+        module: LLVM module
+        box_type: Box type name (e.g., "StringBox", "IntegerBox")
+        args: Constructor arguments
+        dst_vid: Destination value ID for box handle
+        vmap: Value map
+        resolver: Optional resolver for type handling
+    """
+    # Look up or declare the box creation function
+    create_func_name = f"ny_create_{box_type}"
+    create_func = None
+    
+    for f in module.functions:
+        if f.name == create_func_name:
+            create_func = f
+            break
+    
+    if not create_func:
+        # Declare box creation function
+        # Signature depends on box type
+        i64 = ir.IntType(64)
+        i8 = ir.IntType(8)
+        
+        if box_type in ["StringBox", "IntegerBox", "BoolBox"]:
+            # Built-in boxes - default constructors (no args)
+            # Real implementation may have optional args
+            func_type = ir.FunctionType(i64, [])
+        else:
+            # Generic box - variable arguments
+            # For now, assume no args
+            func_type = ir.FunctionType(i64, [])
+        
+        create_func = ir.Function(module, func_type, name=create_func_name)
+    
+    # Prepare arguments
+    call_args = []
+    for i, arg_id in enumerate(args):
+        arg_val = vmap.get(arg_id)
+        
+        if not arg_val:
+            # Default based on box type
+            if box_type == "StringBox":
+                # Empty string
+                i8 = ir.IntType(8)
+                arg_val = ir.Constant(i8.as_pointer(), None)
+            else:
+                # Zero
+                arg_val = ir.Constant(ir.IntType(64), 0)
+        
+        # Type conversion if needed
+        if box_type == "StringBox" and hasattr(arg_val, 'type'):
+            if isinstance(arg_val.type, ir.IntType):
+                # int to string ptr
+                i8 = ir.IntType(8)
+                arg_val = builder.inttoptr(arg_val, i8.as_pointer())
+        
+        call_args.append(arg_val)
+    
+    # Create the box
+    handle = builder.call(create_func, call_args, name=f"new_{box_type}")
+    
+    # Store handle
+    vmap[dst_vid] = handle
+
+def lower_newbox_generic(
+    builder: ir.IRBuilder,
+    module: ir.Module,
+    dst_vid: int,
+    vmap: Dict[int, ir.Value]
+) -> None:
+    """
+    Create a generic box with runtime allocation
+    
+    This is used when box type is not statically known.
+    """
+    # Look up generic allocation function
+    alloc_func = None
+    for f in module.functions:
+        if f.name == "ny_alloc_box":
+            alloc_func = f
+            break
+    
+    if not alloc_func:
+        # Declare ny_alloc_box(size: i64) -> i64
+        i64 = ir.IntType(64)
+        func_type = ir.FunctionType(i64, [i64])
+        alloc_func = ir.Function(module, func_type, name="ny_alloc_box")
+    
+    # Default box size (e.g., 64 bytes)
+    size = ir.Constant(ir.IntType(64), 64)
+    handle = builder.call(alloc_func, [size], name="new_box")
+    
+    vmap[dst_vid] = handle