feat(mir/llvm): Phase 273 P0-P1 DomainPlan→CorePlan + LLVM arg fix

Phase 273 P0-P1: Two-layer plan architecture
- DomainPlan: Pattern-specific knowledge (ScanWithInit)
- CorePlan: Fixed vocabulary (Seq, Loop, If, Effect, Exit)
- ValueId references only (String expressions forbidden)
- Pipeline: Extractor→Normalizer→Verifier→Lowerer

New plan/ module:
- mod.rs: Type definitions, SSOT spec
- normalizer.rs: DomainPlan→CorePlan + ID allocation
- verifier.rs: V1-V6 invariant checks (fail-fast)
- lowerer.rs: CorePlan→MIR (pattern-agnostic)

LLVM fix (ChatGPT):
- function_lower.py: Fix argument reference bug
- Phase 258 index_of_string now PASS on LLVM backend

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

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

src/runner/modes/llvm.rs

@@ -398,6 +398,17 @@ impl NyashRunner {
         }
         #[cfg(all(not(feature = "llvm-inkwell-legacy")))]
         {
+            // 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"
+                );
+                process::exit(1);
+            }
             crate::console_println!("🔧 Mock LLVM Backend Execution:");
             crate::console_println!("   Build with --features llvm-inkwell-legacy for Rust/inkwell backend, or set NYASH_LLVM_OBJ_OUT and NYASH_LLVM_USE_HARNESS=1 for harness.");
             // NamingBox SSOT: Select entry (arity-aware, Main.main → main fallback)