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/mir/builder/control_flow/mod.rs

@@ -60,6 +60,9 @@ pub(in crate::mir::builder) mod normalization;
 // Phase 264: EdgeCFG Fragment API (入口SSOT)
 pub(in crate::mir::builder) mod edgecfg;
 
+// Phase 273 P0: Plan Extractor (Pure) + PlanLowerer SSOT
+pub(in crate::mir::builder) mod plan;
+
 // Phase 140-P4-A: Re-export for loop_canonicalizer SSOT (crate-wide visibility)
 pub(crate) use joinir::detect_skip_whitespace_pattern;