refactor(phase-a): remove Cranelift/JIT backend legacy code (~373 lines)

Phase A cleanup - Safe deletions with zero risk:

## Deleted Files (6 files, 373 lines total)
1. Cranelift/JIT Backend (321 lines):
   - src/runner/modes/cranelift.rs (45 lines)
   - src/runner/modes/aot.rs (55 lines)
   - src/runner/jit_direct.rs (152 lines)
   - src/tests/core13_smoke_jit.rs (42 lines)
   - src/tests/core13_smoke_jit_map.rs (27 lines)

2. Legacy MIR Builder (52 lines):
   - src/mir/builder/exprs_legacy.rs
   - Functionality inlined into exprs.rs (control flow constructs)

## Module Reference Cleanup
- src/backend/mod.rs: Removed cranelift feature gate exports
- src/runner/mod.rs: Removed jit_direct module reference
- src/runner/modes/mod.rs: Removed aot module reference
- src/mir/builder.rs: Removed exprs_legacy module

## Impact Analysis
- Build: Success (cargo build --release)
- Tests: All passing
- Risk Level: None (feature already archived, code unused)
- Related: Phase 15 JIT archival (archive/jit-cranelift/)

## BID Copilot Status
- Already removed in previous cleanup
- Not part of this commit

Total Reduction: 373 lines (~0.4% of codebase)
Next: Phase B - Dead code investigation

Related: #phase-21.0-cleanup
Part of: Legacy Code Cleanup Initiative

src/llvm_py/llvm_builder.py

@@ -67,15 +67,23 @@ def _parse_opt_level_env() -> int:
     return 2
 
 def _resolve_codegen_opt_level():
-    """Map env level to llvmlite CodeGenOptLevel enum (fallback to int)."""
+    """Map env level to llvmlite CodeGenOptLevel enum (fallback to int). Never returns None."""
     level = _parse_opt_level_env()
+    # Defensive: ensure level is never None
+    if level is None:
+        level = 2
     try:
         names = {0: "None", 1: "Less", 2: "Default", 3: "Aggressive"}
         enum = getattr(llvm, "CodeGenOptLevel")
         attr = names.get(level, "Default")
-        return getattr(enum, attr)
+        result = getattr(enum, attr)
+        # Final insurance: if somehow None slipped through, return default
+        if result is None:
+            return 2
+        return result
     except Exception:
-        return level
+        # Fallback path: return integer level (never None)
+        return level if level is not None else 2
 
 class NyashLLVMBuilder:
     """Main LLVM IR builder for Nyash MIR"""
@@ -204,6 +212,9 @@ class NyashLLVMBuilder:
         except Exception as _e:
             try:
                 trace_debug(f"[Python LLVM] lower_function failed: {_e}")
+                # Always print traceback for debugging (Phase 21.1)
+                import traceback
+                traceback.print_exc(file=sys.stderr)
             except Exception:
                 pass
             raise