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/exprs.rs

@@ -207,7 +207,7 @@ impl super::MirBuilder {
                         // Generic static box: lower all static methods into standalone MIR functions (BoxName.method/N)
                         // Note: Metadata clearing is now handled by BoxCompilationContext (箱理論)
                         // See lifecycle.rs for context creation and builder_calls.rs for context swap
-                        self.user_defined_boxes.insert(name.clone());
+                        self.comp_ctx.user_defined_boxes.insert(name.clone());
                         for (method_name, method_ast) in methods.clone() {
                             if let ASTNode::FunctionDeclaration { params, body, .. } = method_ast {
                                 let func_name = format!(
@@ -222,7 +222,7 @@ impl super::MirBuilder {
                                     body.clone(),
                                 )?;
                                 // Index static method for fallback resolution of bare calls
-                                self.static_method_index
+                                self.comp_ctx.static_method_index
                                     .entry(method_name.clone())
                                     .or_insert_with(Vec::new)
                                     .push((name.clone(), params.len()));
@@ -234,7 +234,7 @@ impl super::MirBuilder {
                     }
                 } else {
                     // Instance box: register type and lower instance methods/ctors as functions
-                    self.user_defined_boxes.insert(name.clone());
+                    self.comp_ctx.user_defined_boxes.insert(name.clone());
                     self.build_box_declaration(
                         name.clone(),
                         methods.clone(),
@@ -311,9 +311,9 @@ impl super::MirBuilder {
                 self.type_ctx.value_types
                     .insert(arr_id, super::MirType::Box("ArrayBox".to_string()));
                 // TypeRegistry + trace for deterministic debug
-                self.type_registry
+                self.comp_ctx.type_registry
                     .record_newbox(arr_id, "ArrayBox".to_string());
-                self.type_registry
+                self.comp_ctx.type_registry
                     .record_type(arr_id, super::MirType::Box("ArrayBox".to_string()));
                 type_trace::origin("newbox:ArrayLiteral", arr_id, "ArrayBox");
                 type_trace::ty(
@@ -354,9 +354,9 @@ impl super::MirBuilder {
                     .insert(map_id, "MapBox".to_string());
                 self.type_ctx.value_types
                     .insert(map_id, super::MirType::Box("MapBox".to_string()));
-                self.type_registry
+                self.comp_ctx.type_registry
                     .record_newbox(map_id, "MapBox".to_string());
-                self.type_registry
+                self.comp_ctx.type_registry
                     .record_type(map_id, super::MirType::Box("MapBox".to_string()));
                 type_trace::origin("newbox:MapLiteral", map_id, "MapBox");
                 type_trace::ty(