refactor(mir): Phase 139-P3-B - RoutingDecision を enum 対応 + レガシー削除

- RoutingDecision の missing_caps を Vec<CapabilityTag> に変更（型安全化）
- error_tags は to_tag() メソッドで自動生成
- 全 callsite を enum variant に修正
- capability_tags モジュール（文字列定数群）を完全削除
- 全テスト PASS（型安全性向上を確認）
- フォーマット適用

src/mir/builder/fields.rs

@@ -16,7 +16,12 @@ impl super::MirBuilder {
 
         // Unified members: if object class is known and has a synthetic getter for `field`,
         // rewrite to method call `__get_<field>()`.
-        if let Some(class_name) = self.type_ctx.value_origin_newbox.get(&object_value).cloned() {
+        if let Some(class_name) = self
+            .type_ctx
+            .value_origin_newbox
+            .get(&object_value)
+            .cloned()
+        {
             if let Some(map) = self.comp_ctx.property_getters_by_box.get(&class_name) {
                 if let Some(kind) = map.get(&field) {
                     let mname = match kind {
@@ -53,15 +58,24 @@ impl super::MirBuilder {
 
         // Propagate recorded origin class for this field if any (ValueId-scoped)
         if let Some(class_name) = self
-            .comp_ctx.field_origin_class
+            .comp_ctx
+            .field_origin_class
             .get(&(object_value, field.clone()))
             .cloned()
         {
-            self.type_ctx.value_origin_newbox.insert(field_val, class_name);
-        } else if let Some(base_cls) = self.type_ctx.value_origin_newbox.get(&object_value).cloned() {
+            self.type_ctx
+                .value_origin_newbox
+                .insert(field_val, class_name);
+        } else if let Some(base_cls) = self
+            .type_ctx
+            .value_origin_newbox
+            .get(&object_value)
+            .cloned()
+        {
             // Cross-function heuristic: use class-level field origin mapping
             if let Some(fcls) = self
-                .comp_ctx.field_origin_by_box
+                .comp_ctx
+                .field_origin_by_box
                 .get(&(base_cls.clone(), field.clone()))
                 .cloned()
             {
@@ -78,8 +92,11 @@ impl super::MirBuilder {
         }
 
         // If base is a known newbox and field is weak, emit WeakLoad (+ optional barrier)
-        let mut inferred_class: Option<String> =
-            self.type_ctx.value_origin_newbox.get(&object_value).cloned();
+        let mut inferred_class: Option<String> = self
+            .type_ctx
+            .value_origin_newbox
+            .get(&object_value)
+            .cloned();
         if inferred_class.is_none() {
             if let ASTNode::FieldAccess {
                 object: inner_obj,
@@ -89,7 +106,8 @@ impl super::MirBuilder {
             {
                 if let Ok(base_id) = self.build_expression(*inner_obj.clone()) {
                     if let Some(cls) = self
-                        .comp_ctx.field_origin_class
+                        .comp_ctx
+                        .field_origin_class
                         .get(&(base_id, inner_field))
                         .cloned()
                     {
@@ -128,7 +146,12 @@ impl super::MirBuilder {
         value_result = self.local_arg(value_result);
 
         // If base is known and field is weak, create WeakRef before store
-        if let Some(class_name) = self.type_ctx.value_origin_newbox.get(&object_value).cloned() {
+        if let Some(class_name) = self
+            .type_ctx
+            .value_origin_newbox
+            .get(&object_value)
+            .cloned()
+        {
             if let Some(weak_set) = self.comp_ctx.weak_fields_by_box.get(&class_name) {
                 if weak_set.contains(&field) {
                     value_result = self.emit_weak_new(value_result)?;
@@ -158,7 +181,12 @@ impl super::MirBuilder {
         })?;
 
         // Write barrier if weak field
-        if let Some(class_name) = self.type_ctx.value_origin_newbox.get(&object_value).cloned() {
+        if let Some(class_name) = self
+            .type_ctx
+            .value_origin_newbox
+            .get(&object_value)
+            .cloned()
+        {
             if let Some(weak_set) = self.comp_ctx.weak_fields_by_box.get(&class_name) {
                 if weak_set.contains(&field) {
                     let _ = self.emit_barrier_write(value_result);
@@ -167,12 +195,24 @@ impl super::MirBuilder {
         }
 
         // Record origin class for this field value if known
-        if let Some(val_cls) = self.type_ctx.value_origin_newbox.get(&value_result).cloned() {
-            self.comp_ctx.field_origin_class
+        if let Some(val_cls) = self
+            .type_ctx
+            .value_origin_newbox
+            .get(&value_result)
+            .cloned()
+        {
+            self.comp_ctx
+                .field_origin_class
                 .insert((object_value, field.clone()), val_cls.clone());
             // Also record class-level mapping if base object class is known
-            if let Some(base_cls) = self.type_ctx.value_origin_newbox.get(&object_value).cloned() {
-                self.comp_ctx.field_origin_by_box
+            if let Some(base_cls) = self
+                .type_ctx
+                .value_origin_newbox
+                .get(&object_value)
+                .cloned()
+            {
+                self.comp_ctx
+                    .field_origin_by_box
                     .insert((base_cls, field.clone()), val_cls);
             }
         }