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/decls.rs

@@ -26,8 +26,8 @@ impl super::MirBuilder {
             }
         }
         // Within this lowering, treat `me` receiver as this static box
-        let saved_static = self.current_static_box.clone();
-        self.current_static_box = Some(box_name.clone());
+        let saved_static = self.comp_ctx.current_static_box.clone();
+        self.comp_ctx.current_static_box = Some(box_name.clone());
         // Look for the main() method
         let out = if let Some(main_method) = methods.get("main") {
             if let ASTNode::FunctionDeclaration { params, body, .. } = main_method {
@@ -111,7 +111,7 @@ impl super::MirBuilder {
                     }
                     self.variable_ctx.variable_map.insert(p.clone(), pid);
                     // 関数スコープ SlotRegistry にも登録しておくよ（観測専用）
-                    if let Some(reg) = self.current_slot_registry.as_mut() {
+                    if let Some(reg) = self.comp_ctx.current_slot_registry.as_mut() {
                         let ty = self.type_ctx.value_types.get(&pid).cloned();
                         reg.ensure_slot(p, ty);
                     }
@@ -121,13 +121,13 @@ impl super::MirBuilder {
                     "[build_static_main_box] Storing fn_body_ast with {} nodes for inline main()",
                     body.len()
                 );
-                self.fn_body_ast = Some(body.clone());
+                self.comp_ctx.fn_body_ast = Some(body.clone());
 
                 // Lower statements in order to preserve def→use
                 let lowered = self.cf_block(body.clone());
 
                 // Phase 200-C: Clear fn_body_ast after main() lowering
-                self.fn_body_ast = None;
+                self.comp_ctx.fn_body_ast = None;
 
                 self.variable_ctx.variable_map = saved_var_map;
                 lowered
@@ -138,7 +138,7 @@ impl super::MirBuilder {
             Err("static box must contain a main() method".to_string())
         };
         // Restore static box context
-        self.current_static_box = saved_static;
+        self.comp_ctx.current_static_box = saved_static;
         out
     }
 
@@ -164,7 +164,7 @@ impl super::MirBuilder {
         // Record weak fields for this box
         if !weak_fields.is_empty() {
             let set: HashSet<String> = weak_fields.into_iter().collect();
-            self.weak_fields_by_box.insert(name.clone(), set);
+            self.comp_ctx.weak_fields_by_box.insert(name.clone(), set);
         }
 
         // Reserve method slots for user-defined instance methods (deterministic, starts at 4)
@@ -206,7 +206,7 @@ impl super::MirBuilder {
                 if let Some((k, prop)) = kind_and_prop {
                     use std::collections::HashMap;
                     let entry: &mut HashMap<String, super::PropertyKind> = self
-                        .property_getters_by_box
+                        .comp_ctx.property_getters_by_box
                         .entry(name.clone())
                         .or_insert_with(HashMap::new);
                     entry.insert(prop, k);