refactor(mir): Remove CompilationContext legacy fields (Phase 2-7/7) 🎉

Phase 2 完全完了！全 7 Context のレガシーフィールドを完全削除。

## Changes
- Migrated all access sites to comp_ctx.* (15 fields)
- Removed 15 deprecated fields:
  * compilation_context
  * current_static_box
  * user_defined_boxes
  * reserved_value_ids
  * fn_body_ast
  * weak_fields_by_box
  * property_getters_by_box
  * field_origin_class
  * field_origin_by_box
  * static_method_index
  * method_tail_index
  * method_tail_index_source_len
  * type_registry
  * current_slot_registry
  * plugin_method_sigs
- Removed initialization code (15 field inits)

## Phase 2 完了！🎉
- builder.rs: 1222 → 1127 lines (-95 lines net)
- Deprecation warnings: 86 → 0 (完全排除)
- 全 36 deprecated fields 削除完了
- 全 14 sync helpers 削除完了
- 7 Context 完全SSOT化

## Tests
- cargo build --release: SUCCESS
- cargo test --release --lib: 1033/1033 PASS ✅
- Deprecation warnings: 0 ✅

Phase 2 Progress: 7/7 contexts complete (100%) ✅
- ✅ MetadataContext
- ✅ CoreContext
- ✅ TypeContext
- ✅ ScopeContext
- ✅ BindingContext
- ✅ VariableContext
- ✅ CompilationContext (this commit) 🎉

Phase 136 Context Box化: 完全完了！

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

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

src/mir/builder/fields.rs

@@ -17,7 +17,7 @@ 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(map) = self.property_getters_by_box.get(&class_name) {
+            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 {
                         super::PropertyKind::Computed => format!("__get_{}", field),
@@ -53,7 +53,7 @@ impl super::MirBuilder {
 
         // Propagate recorded origin class for this field if any (ValueId-scoped)
         if let Some(class_name) = self
-            .field_origin_class
+            .comp_ctx.field_origin_class
             .get(&(object_value, field.clone()))
             .cloned()
         {
@@ -61,7 +61,7 @@ impl super::MirBuilder {
         } 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
-                .field_origin_by_box
+                .comp_ctx.field_origin_by_box
                 .get(&(base_cls.clone(), field.clone()))
                 .cloned()
             {
@@ -89,7 +89,7 @@ impl super::MirBuilder {
             {
                 if let Ok(base_id) = self.build_expression(*inner_obj.clone()) {
                     if let Some(cls) = self
-                        .field_origin_class
+                        .comp_ctx.field_origin_class
                         .get(&(base_id, inner_field))
                         .cloned()
                     {
@@ -99,7 +99,7 @@ impl super::MirBuilder {
             }
         }
         if let Some(class_name) = inferred_class {
-            if let Some(weak_set) = self.weak_fields_by_box.get(&class_name) {
+            if let Some(weak_set) = self.comp_ctx.weak_fields_by_box.get(&class_name) {
                 if weak_set.contains(&field) {
                     let loaded = self.emit_weak_load(field_val)?;
                     let _ = self.emit_barrier_read(loaded);
@@ -129,7 +129,7 @@ impl super::MirBuilder {
 
         // 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(weak_set) = self.weak_fields_by_box.get(&class_name) {
+            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)?;
                 }
@@ -159,7 +159,7 @@ 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(weak_set) = self.weak_fields_by_box.get(&class_name) {
+            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);
                 }
@@ -168,11 +168,11 @@ 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.field_origin_class
+            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.field_origin_by_box
+                self.comp_ctx.field_origin_by_box
                     .insert((base_cls, field.clone()), val_cls);
             }
         }