refactor(control_flow): Phase 134 P0 - Normalization entry point SSOT

Consolidate dual entry points into unified NormalizationPlan system:

Problem:
- Dual entry points: try_normalized_shadow() + suffix_router_box
- ~220 lines of duplicated pattern detection logic
- Maintenance burden: changes required in two places

Solution:
- New normalization module with Box-First architecture
- NormalizationPlanBox: Single source of truth for pattern detection
- NormalizationExecuteBox: Single source of truth for execution

Implementation:
- src/mir/builder/control_flow/normalization/ (new module)
  - README.md: Design contract (SSOT)
  - plan.rs: NormalizationPlan data structure
  - plan_box.rs: Pattern detection (7 unit tests)
  - execute_box.rs: Execution logic
  - mod.rs: Module integration

Refactored files:
- routing.rs::try_normalized_shadow(): 165 → 87 lines (-78 lines)
- suffix_router_box::try_lower_loop_suffix(): 258 → 116 lines (-142 lines)

Pattern support maintained:
- Phase 131: loop(true) only (consumed: 1)
- Phase 132: loop + single post (consumed: 3)
- Phase 133: loop + multiple post (consumed: 2+N)

Box-First principles:
- Plan Box: Detection responsibility only
- Execute Box: Execution responsibility only
- README.md: Contract documentation (SSOT)
- Clear separation enables independent testing

Test results:
- cargo test --lib: 1186 PASS (10 new tests added)
- Phase 133 VM/LLVM EXE: PASS (exit code 6)
- Phase 132 LLVM EXE: PASS (exit code 3)
- Phase 131 LLVM EXE: PASS (exit code 1)

Benefits:
- Code duplication eliminated (~220 lines)
- Single source of truth for normalization decisions
- Improved maintainability and testability
- Future-proof extensibility (easy to add new patterns)

Default behavior unchanged: Dev-only guard maintained

Related: Phase 134 normalization infrastructure improvement

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

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

src/mir/builder/control_flow/mod.rs

@@ -54,6 +54,9 @@ pub(in crate::mir::builder) mod exception;
 // Phase 6: Utility functions
 pub(in crate::mir::builder) mod utils;
 
+// Phase 134 P0: Normalization entry point consolidation
+pub(in crate::mir::builder) mod normalization;
+
 // Phase 140-P4-A: Re-export for loop_canonicalizer SSOT (crate-wide visibility)
 pub(crate) use joinir::{detect_skip_whitespace_pattern, SkipWhitespaceInfo};