refactor(llvm): Phase 286B - Unify error handling with Result<T, LlvmRunError>

This refactoring consolidates 20+ duplicated error handling patterns in LLVM mode
into a unified Result-based error handling system.

New files:
- error.rs: LlvmRunError type with code, msg, and convenience constructors
- report.rs: emit_error_and_exit() - single exit point for all LLVM errors

Changes:
- harness_executor.rs: Returns Result<i32, LlvmRunError> instead of Option<i32>
- pyvm_executor.rs: Returns Result<i32, LlvmRunError> instead of Option<i32>
- fallback_executor.rs: Returns Result<i32, LlvmRunError> instead of i32
- mod.rs: Updated all error handling to use report::emit_error_and_exit()

Benefits:
1. Unified error handling with Result<T, LlvmRunError>
2. Clear separation: Executors return errors, orchestrator handles exit
3. Single exit point guarantees leak report consistency
4. Reduced code duplication (20+ patterns consolidated)

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

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

src/runner/modes/llvm/fallback_executor.rs

@@ -3,12 +3,13 @@
 //! Handles fallback execution when LLVM backends are not available.
 
 use nyash_rust::{mir::MirModule, mir::MirInstruction};
+use super::error::LlvmRunError;
 
 /// Fallback executor Box
 ///
 /// **Responsibility**: Execute fallback path (feature check + mock)
 /// **Input**: &MirModule
-/// **Output**: i32 (exit code)
+/// **Output**: Result<i32, LlvmRunError> (Ok(exit_code) on success, Err on failure)
 pub struct FallbackExecutorBox;
 
 impl FallbackExecutorBox {
@@ -20,18 +21,14 @@ impl FallbackExecutorBox {
     ///
     /// Otherwise, executes mock execution that inspects the MIR
     /// and returns a deterministic exit code based on Return instructions.
-    pub fn execute(module: &MirModule) -> i32 {
+    pub fn execute(module: &MirModule) -> Result<i32, LlvmRunError> {
         // 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;
+            return Err(LlvmRunError::fatal(
+                "LLVM harness requested (NYASH_LLVM_USE_HARNESS=1), but this binary was built without `--features llvm` (llvm-harness). Fix: cargo build --release --features llvm"
+            ));
         }
 
         crate::console_println!("🔧 Mock LLVM Backend Execution:");
@@ -46,11 +43,11 @@ impl FallbackExecutorBox {
                     match inst {
                         MirInstruction::Return { value: Some(_) } => {
                             crate::console_println!("✅ Mock exit code: 42");
-                            return 42;
+                            return Ok(42);
                         }
                         MirInstruction::Return { value: None } => {
                             crate::console_println!("✅ Mock exit code: 0");
-                            return 0;
+                            return Ok(0);
                         }
                         _ => {}
                     }
@@ -59,6 +56,6 @@ impl FallbackExecutorBox {
         }
 
         crate::console_println!("✅ Mock exit code: 0");
-        0
+        Ok(0)
     }
 }