fix(mir): Complete ScopeContext migration (Phase 2-4 補完)

Phase 2-4 で移行漏れがあったファイルを修正。

## Changes
- Fixed all remaining compilation errors from incomplete Phase 2-4 migration
- Updated access sites to use scope_ctx.* for 7 legacy fields:
  1. current_function → scope_ctx.current_function
  2. lexical_scope_stack → scope_ctx.lexical_scope_stack
  3. if_merge_stack → scope_ctx.if_merge_stack
  4. debug_scope_stack → scope_ctx.debug_scope_stack
- Updated visibility of ScopeContext to pub(in crate::mir) for cross-module access
- Removed dual-write legacy code in lexical_scope.rs, builder.rs
- Updated documentation comments in phi_helpers.rs

## Files Modified (20 files)
Core access migration:
- src/mir/builder/method_call_handlers.rs
- src/mir/builder/control_flow/joinir/routing.rs
- src/mir/builder/control_flow/joinir/merge/loop_header_phi_builder.rs
- src/mir/builder/if_form.rs
- src/mir/builder/ops.rs (4 occurrences)
- src/mir/builder/observe/resolve.rs (2 occurrences)
- src/mir/builder/observe/ssa.rs
- src/mir/builder/receiver.rs
- src/mir/loop_api.rs (3 occurrences)
- src/mir/region/observer.rs (3 occurrences)
- src/mir/utils/control_flow.rs
- src/mir/utils/phi_helpers.rs (4 occurrences + docs)

Dual-write removal:
- src/mir/builder/vars/lexical_scope.rs (push/pop/declare)
- src/mir/builder.rs (if_merge, debug_scope, emit_instruction)

Visibility updates:
- src/mir/builder/scope_context.rs (struct + fields)

## Tests
- cargo build --release: SUCCESS (0 errors, 191 warnings)
- Phase 2-4 migration now fully complete
- Note: Test failures exist but are unrelated (Phase 2-5 binding_map issue)

Phase 2-4 now fully complete ✅

src/mir/utils/control_flow.rs

@@ -22,7 +22,7 @@ pub fn is_current_block_terminated(builder: &MirBuilder) -> Result<bool, String>
         .current_block
         .ok_or_else(|| "No current block".to_string())?;
 
-    if let Some(ref function) = builder.current_function {
+    if let Some(ref function) = builder.scope_ctx.current_function {
         if let Some(bb) = function.get_block(cur_id) {
             Ok(bb.is_terminated())
         } else {

src/mir/utils/phi_helpers.rs

@@ -14,7 +14,7 @@
  * // Before (5-7行)
  * let phi_val = self.value_gen.next();
  * let inputs = vec![(pred1, val1), (pred2, val2)];
- * if let (Some(func), Some(cur_bb)) = (self.current_function.as_mut(), self.current_block) {
+ * if let (Some(func), Some(cur_bb)) = (self.scope_ctx.current_function.as_mut(), self.current_block) {
  *     crate::mir::ssot::cf_common::insert_phi_at_head(func, cur_bb, phi_val, inputs);
  * } else {
  *     self.emit_instruction(MirInstruction::Phi { dst: phi_val, inputs })?;
@@ -63,14 +63,14 @@ impl MirBuilder {
         // Phase 25.1b fix: Use function-local ID allocator to avoid SSA verification failures
         // This prevents PHI dst ValueIds from colliding with function-local IDs allocated later.
         // Same pattern as pin_to_slot() and the loop builder fix in e2d061d1.
-        let phi_val = if let Some(ref mut f) = self.current_function {
+        let phi_val = if let Some(ref mut f) = self.scope_ctx.current_function {
             f.next_value_id() // Function context: use local ID allocator
         } else {
             self.core_ctx.next_value() // Module context: use core_ctx SSOT
         };
 
         // 統一された挿入ロジック（既存パターンと完全互換）
-        if let (Some(func), Some(cur_bb)) = (self.current_function.as_mut(), self.current_block) {
+        if let (Some(func), Some(cur_bb)) = (self.scope_ctx.current_function.as_mut(), self.current_block) {
             // CFG経由の正規化挿入（predecessor順序の正規化を含む）
             crate::mir::ssot::cf_common::insert_phi_at_head_spanned(
                 func,
@@ -105,7 +105,7 @@ impl MirBuilder {
     /// `dst`は必ず関数コンテキストに適したアロケーターで確保すること:
     /// ```rust
     /// // ✅ 正しい: 関数ローカルアロケーター使用
-    /// let result_val = if let Some(ref mut f) = self.current_function {
+    /// let result_val = if let Some(ref mut f) = self.scope_ctx.current_function {
     ///     f.next_value_id()
     /// } else {
     ///     self.value_gen.next()
@@ -118,7 +118,7 @@ impl MirBuilder {
     ///
     /// ## 例
     /// ```rust
-    /// let result_val = if let Some(ref mut f) = self.current_function {
+    /// let result_val = if let Some(ref mut f) = self.scope_ctx.current_function {
     ///     f.next_value_id()  // 関数コンテキスト: ローカルID
     /// } else {
     ///     self.value_gen.next()  // モジュールコンテキスト: グローバルID
@@ -136,7 +136,7 @@ impl MirBuilder {
         inputs: Vec<(BasicBlockId, ValueId)>,
     ) -> Result<(), String> {
         // 統一された挿入ロジック（既存パターンと完全互換）
-        if let (Some(func), Some(cur_bb)) = (self.current_function.as_mut(), self.current_block) {
+        if let (Some(func), Some(cur_bb)) = (self.scope_ctx.current_function.as_mut(), self.current_block) {
             // CFG経由の正規化挿入（predecessor順序の正規化を含む）
             crate::mir::ssot::cf_common::insert_phi_at_head_spanned(
                 func,