Phase 22.x WIP: LLVM backend improvements + MIR builder enhancements

LLVM backend improvements:
- Add native LLVM backend support (NYASH_LLVM_BACKEND=native)
- Add crate backend selector with priority (crate > llvmlite)
- Add native_llvm_builder.py for native IR generation
- Add NYASH_LLVM_NATIVE_TRACE=1 for IR dump

MIR builder enhancements:
- Refactor lower_if_compare_* boxes for better code generation
- Refactor lower_return_* boxes for optimized returns
- Refactor lower_loop_* boxes for loop handling
- Refactor lower_method_* boxes for method calls
- Update pattern_util_box for better pattern matching

Smoke tests:
- Add phase2100 S3 backend selector tests (17 new tests)
- Add phase2120 native backend tests (4 new tests)
- Add phase2034 MIR builder internal tests (2 new tests)
- Add phase2211 TLV shim parity test

Documentation:
- Update ENV_VARS.md with LLVM backend variables
- Update CURRENT_TASK.md with progress
- Update README.md and CHANGELOG.md

Config:
- Add NYASH_LLVM_BACKEND env support in src/config/env.rs
- Update ny_mir_builder.sh for backend selection
- Update dispatch.rs for backend routing

Tools:
- Add tools/native_llvm_builder.py
- Update smokes/v2/profiles/quick/core/phase2100/run_all.sh

Known: Many Hako builder internal files modified for optimization

src/llvm_py/builders/function_lower.py

@@ -24,8 +24,8 @@ def lower_function(builder, func_data: Dict[str, Any]):
 
     # Determine function signature
     if name == "ny_main":
-        # Special case: ny_main returns i32
-        func_ty = ir.FunctionType(builder.i32, [])
+        # Special case: ny_main returns i64 to match runtime (nyrt) expectations
+        func_ty = ir.FunctionType(builder.i64, [])
     else:
         # Default: i64(i64, ...) signature; derive arity from '/N' suffix when params missing
         m = re.search(r"/(\d+)$", name)

src/llvm_py/instructions/binop.py

@@ -45,7 +45,23 @@ def lower_binop(
         lhs_val = ir.Constant(ir.IntType(64), 0)
     if rhs_val is None:
         rhs_val = ir.Constant(ir.IntType(64), 0)
-    
+    # Normalize operation aliases (textual -> symbolic)
+    op_raw = op or ''
+    op_l = op_raw.lower()
+    alias = {
+        'add': '+', 'plus': '+',
+        'sub': '-', 'minus': '-',
+        'mul': '*', 'times': '*',
+        'div': '/',
+        'mod': '%', 'rem': '%',
+        'band': '&', 'bitand': '&',
+        'bor': '|', 'bitor': '|',
+        'bxor': '^', 'xor': '^',
+        'shl': '<<',
+        'shr': '>>', 'ashr': '>>',
+    }
+    op = alias.get(op_l, op_raw)
+
     # Relational/equality operators delegate to compare
     if op in ('==','!=','<','>','<=','>='):
         # Delegate to compare with resolver/preds context to maintain dominance via localization

src/llvm_py/instructions/ret.py

@@ -97,9 +97,16 @@ def lower_return(
                 # Pointer type - null
                 ret_val = ir.Constant(return_type, None)
 
-        # If still zero-like and we have predecessor snapshots, synthesize a minimal PHI at block head.
+        # If still zero-like (typed zero) and we have predecessor snapshots, synthesize a minimal PHI at block head.
         try:
-            zero_like = isinstance(ret_val, ir.Constant)
+            zero_like = False
+            if isinstance(ret_val, ir.Constant):
+                if isinstance(return_type, ir.IntType):
+                    zero_like = (str(ret_val) == str(ir.Constant(return_type, 0)))
+                elif isinstance(return_type, ir.DoubleType):
+                    zero_like = (str(ret_val) == str(ir.Constant(return_type, 0.0)))
+                elif isinstance(return_type, ir.PointerType):
+                    zero_like = (str(ret_val) == str(ir.Constant(return_type, None)))
             if zero_like and preds is not None and block_end_values is not None and bb_map is not None and isinstance(value_id, int):
                 # Derive current block id from name like 'bb3'
                 cur_bid = None

src/llvm_py/llvm_builder.py

@@ -152,7 +152,8 @@ class NyashLLVMBuilder:
             else:
                 arity = int(m.group(1)) if m else len(params_list)
             if name == "ny_main":
-                fty = ir.FunctionType(self.i32, [])
+                # Align with runtime expectation: ny_main returns i64
+                fty = ir.FunctionType(self.i64, [])
             else:
                 fty = ir.FunctionType(self.i64, [self.i64] * arity)
             exists = False