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/utils.rs

@@ -46,7 +46,7 @@ impl super::MirBuilder {
             };
 
             // Phase 201-A: Skip reserved PHI dst ValueIds
-            if !self.reserved_value_ids.contains(&candidate) {
+            if !self.comp_ctx.reserved_value_ids.contains(&candidate) {
                 return candidate;
             }
             // Reserved ID - try next one (loop continues)
@@ -327,7 +327,7 @@ impl super::MirBuilder {
                 }
             }
             if let Some(bt) = recv_box {
-                if let Some(mt) = self.plugin_method_sigs.get(&(bt.clone(), method.clone())) {
+                if let Some(mt) = self.comp_ctx.plugin_method_sigs.get(&(bt.clone(), method.clone())) {
                     self.type_ctx.value_types.insert(d, mt.clone());
                 } else {
                     // Phase 84-4-B: ビルトイン Box のメソッド戻り値型推論