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/control_flow/joinir/merge/mod.rs

@@ -295,11 +295,11 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
 
     // Phase 201-A: Set reserved IDs in MirBuilder so next_value_id() skips them
     // This protects against carrier corruption when break conditions emit Const instructions
-    builder.reserved_value_ids = reserved_phi_dsts.clone();
-    if debug && !builder.reserved_value_ids.is_empty() {
+    builder.comp_ctx.reserved_value_ids = reserved_phi_dsts.clone();
+    if debug && !builder.comp_ctx.reserved_value_ids.is_empty() {
         eprintln!(
-            "[cf_loop/joinir] Phase 201-A: Set builder.reserved_value_ids = {:?}",
-            builder.reserved_value_ids
+            "[cf_loop/joinir] Phase 201-A: Set builder.comp_ctx.reserved_value_ids = {:?}",
+            builder.comp_ctx.reserved_value_ids
         );
     }
 
@@ -750,14 +750,14 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
 
     // Phase 201-A: Clear reserved ValueIds after merge completes
     // Future loops will set their own reserved IDs
-    if !builder.reserved_value_ids.is_empty() {
+    if !builder.comp_ctx.reserved_value_ids.is_empty() {
         if debug {
             eprintln!(
                 "[cf_loop/joinir] Phase 201-A: Clearing reserved_value_ids (was {:?})",
-                builder.reserved_value_ids
+                builder.comp_ctx.reserved_value_ids
             );
         }
-        builder.reserved_value_ids.clear();
+        builder.comp_ctx.reserved_value_ids.clear();
     }
 
     // Phase 246-EX-FIX: Handle loop variable expr_result separately from carrier expr_result