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/stmts.rs

@@ -340,7 +340,7 @@ impl super::MirBuilder {
             }
             self.declare_local_in_current_scope(var_name, var_id)?;
             // SlotRegistry にもローカル変数スロットを登録しておくよ（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(&var_id).cloned();
                 reg.ensure_slot(&var_name, ty);
             }
@@ -430,7 +430,7 @@ impl super::MirBuilder {
             self.type_ctx.value_types
                 .insert(future_id, MirType::Future(Box::new(MirType::Unknown)));
             self.variable_ctx.variable_map.insert(variable.clone(), future_id);
-            if let Some(reg) = self.current_slot_registry.as_mut() {
+            if let Some(reg) = self.comp_ctx.current_slot_registry.as_mut() {
                 reg.ensure_slot(&variable, None);
             }
             return Ok(future_id);
@@ -449,7 +449,7 @@ impl super::MirBuilder {
         self.type_ctx.value_types
             .insert(future_id, MirType::Future(Box::new(inner)));
         self.variable_ctx.variable_map.insert(variable.clone(), future_id);
-        if let Some(reg) = self.current_slot_registry.as_mut() {
+        if let Some(reg) = self.comp_ctx.current_slot_registry.as_mut() {
             reg.ensure_slot(&variable, None);
         }
         Ok(future_id)
@@ -481,14 +481,14 @@ impl super::MirBuilder {
         if let Some(id) = self.variable_ctx.variable_map.get("me").cloned() {
             return Ok(id);
         }
-        let me_tag = if let Some(ref cls) = self.current_static_box {
+        let me_tag = if let Some(ref cls) = self.comp_ctx.current_static_box {
             cls.clone()
         } else {
             "__me__".to_string()
         };
         let me_value = crate::mir::builder::emission::constant::emit_string(self, me_tag);
         self.variable_ctx.variable_map.insert("me".to_string(), me_value);
-        if let Some(reg) = self.current_slot_registry.as_mut() {
+        if let Some(reg) = self.comp_ctx.current_slot_registry.as_mut() {
             reg.ensure_slot("me", None);
         }
         // P0: Known 化 — 分かる範囲で me の起源クラスを付与（挙動不変）。