refactor(llvm): Phase 286 - Modularize llvm.rs into 10 boxes

Following Phase 33 success pattern:
- Single responsibility per box
- Testability through isolated boxes
- Reusability across backends

Before: 449 lines monolithic function
After: 229 line orchestrator + 10 focused boxes

Boxes created:
- plugin_init.rs (plugin initialization)
- using_resolver.rs (using/prelude handling)
- mir_compiler.rs (AST → MIR compilation)
- method_id_injector.rs (method_id injection)
- joinir_experiment.rs (JoinIR experiment, feature-gated)
- pyvm_executor.rs (PyVM harness, dev/test)
- object_emitter.rs (LLVM object emit)
- harness_executor.rs (LLVM harness execution)
- exit_reporter.rs (leak report, Phase 285LLVM-0)
- fallback_executor.rs (mock/legacy execution)

Test Results:
- ✅ cargo build --release
- ✅ cargo build --release --features llvm
- ✅ 18 VM/LLVM parity tests PASS
- ✅ No regressions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/runner/modes/llvm/fallback_executor.rs

@@ -0,0 +1,64 @@
+//! Fallback executor for LLVM mode (mock/legacy)
+//!
+//! Handles fallback execution when LLVM backends are not available.
+
+use nyash_rust::{mir::MirModule, mir::MirInstruction};
+
+/// Fallback executor Box
+///
+/// **Responsibility**: Execute fallback path (feature check + mock)
+/// **Input**: &MirModule
+/// **Output**: i32 (exit code)
+pub struct FallbackExecutorBox;
+
+impl FallbackExecutorBox {
+    /// Execute fallback path (feature check + mock)
+    ///
+    /// Fail-fast: if the user explicitly requested the llvmlite harness
+    /// but this binary was built without the `llvm-harness` feature,
+    /// do not silently fall back to mock.
+    ///
+    /// Otherwise, executes mock execution that inspects the MIR
+    /// and returns a deterministic exit code based on Return instructions.
+    pub fn execute(module: &MirModule) -> i32 {
+        // Fail-fast: if the user explicitly requested the llvmlite harness
+        // but this binary was built without the `llvm-harness` feature,
+        // do not silently fall back to mock.
+        if crate::config::env::env_bool("NYASH_LLVM_USE_HARNESS") {
+            crate::console_println!(
+                "❌ LLVM harness requested (NYASH_LLVM_USE_HARNESS=1), but this binary was built without `--features llvm` (llvm-harness)."
+            );
+            crate::console_println!(
+                "   Fix: cargo build --release --features llvm"
+            );
+            return 1;
+        }
+
+        crate::console_println!("🔧 Mock LLVM Backend Execution:");
+        crate::console_println!("   Build with --features llvm for real backend.");
+
+        // NamingBox SSOT: Select entry (arity-aware, Main.main → main fallback)
+        let entry = crate::runner::modes::common_util::entry_selection::select_entry_function(module);
+
+        if let Some(main_func) = module.functions.get(&entry) {
+            for (_bid, block) in &main_func.blocks {
+                for inst in &block.instructions {
+                    match inst {
+                        MirInstruction::Return { value: Some(_) } => {
+                            crate::console_println!("✅ Mock exit code: 42");
+                            return 42;
+                        }
+                        MirInstruction::Return { value: None } => {
+                            crate::console_println!("✅ Mock exit code: 0");
+                            return 0;
+                        }
+                        _ => {}
+                    }
+                }
+            }
+        }
+
+        crate::console_println!("✅ Mock exit code: 0");
+        0
+    }
+}