refactor(phi_core): Phase 191 loopform_builder.rs modularization

- loopform_context.rs: ValueId management (148 lines)
- loopform_variable_models.rs: Type definitions (101 lines)
- loopform_utils.rs: Utilities (112 lines)
- loopform_passes.rs: 4-pass architecture docs (133 lines)
- loopform_exit_phi.rs: Exit PHI builder (96 lines)
- loopform_builder.rs: Reduced from 1278 → 1166 lines (8.9% reduction)

Total new modules: 590 lines
Responsibility separation achieved
Test visibility improved
All tests passing (loop_min_while.hako verified)

src/mir/phi_core/mod.rs

@@ -12,6 +12,13 @@ pub mod conservative;
 // Phase 84-5: if_phi 削除（レガシーフォールバック完全削除）
 // Phase 30 F-2.1: loop_phi 削除（LoopFormBuilder が SSOT）
 pub mod loop_snapshot_merge;
+
+// Phase 191: LoopForm modularization
+pub mod loopform_context;
+pub mod loopform_variable_models;
+pub mod loopform_utils;
+pub mod loopform_passes;
+pub mod loopform_exit_phi;
 pub mod loopform_builder;
 // Trio legacy boxes removed in Phase 70: LoopScopeShape now owns classification/liveness.