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>

crates/nyash_kernel/src/lib.rs

@@ -828,7 +828,9 @@ pub extern "C" fn main() -> i32 {
                     if let Some(ib) = obj.as_any().downcast_ref::<IntegerBox>() {
                         ib.value as i64
                     } else {
-                        0
+                        // Avoid “raw integer vs handle id” collision:
+                        // if the handle exists but isn't an IntegerBox, treat `v` as a raw i64.
+                        v
                     }
                 } else {
                     v