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/builder.rs

@@ -356,17 +356,13 @@ impl MirBuilder {
     }
 
     /// Push/pop helpers for If merge context (best-effort; optional usage)
-    #[allow(deprecated)]
     pub(super) fn push_if_merge(&mut self, bb: BasicBlockId) {
-        // Phase 136 Step 3/7: Update both scope_ctx (SSOT) and legacy field (backward compat)
+        // Phase 2-4: Use scope_ctx only (legacy field removed)
         self.scope_ctx.push_if_merge(bb);
-        self.if_merge_stack.push(bb);
     }
-    #[allow(deprecated)]
     pub(super) fn pop_if_merge(&mut self) {
-        // Phase 136 Step 3/7: Update both scope_ctx (SSOT) and legacy field (backward compat)
+        // Phase 2-4: Use scope_ctx only (legacy field removed)
         let _ = self.scope_ctx.pop_if_merge();
-        let _ = self.if_merge_stack.pop();
     }
 
     /// Suppress entry pin copy for the next start_new_block (used for merge blocks).
@@ -427,20 +423,16 @@ impl MirBuilder {
     }
 
     #[inline]
-    #[allow(deprecated)]
     pub(crate) fn debug_push_region<S: Into<String>>(&mut self, region: S) {
-        // Phase 136 Step 3/7: Update both scope_ctx (SSOT) and legacy field (backward compat)
+        // Phase 2-4: Use scope_ctx only (legacy field removed)
         let region = region.into();
-        self.scope_ctx.debug_push_region(region.clone());
+        self.scope_ctx.debug_push_region(region);
-        self.debug_scope_stack.push(region);
     }
 
     #[inline]
-    #[allow(deprecated)]
     pub(crate) fn debug_pop_region(&mut self) {
-        // Phase 136 Step 3/7: Update both scope_ctx (SSOT) and legacy field (backward compat)
+        // Phase 2-4: Use scope_ctx only (legacy field removed)
         self.scope_ctx.debug_pop_region();
-        let _ = self.debug_scope_stack.pop();
     }
 
     #[inline]
@@ -704,7 +696,7 @@ impl MirBuilder {
 
         // Precompute debug metadata to avoid borrow conflicts later
         let _dbg_fn_name = self
-            .current_function
+            .scope_ctx.current_function
             .as_ref()
             .map(|f| f.signature.name.clone());
         let _dbg_region_id = self.debug_current_region_id();
@@ -755,7 +747,7 @@ impl MirBuilder {
             }
         }
 
-        if let Some(ref mut function) = self.current_function {
+        if let Some(ref mut function) = self.scope_ctx.current_function {
             // Pre-capture branch/jump targets for predecessor update after we finish
             // mutably borrowing the current block.
             let (then_t, else_t, jump_t) = match &instruction {
@@ -918,7 +910,7 @@ impl MirBuilder {
         phi_id: ValueId,
         new_inputs: Vec<(BasicBlockId, ValueId)>,
     ) -> Result<(), String> {
-        if let Some(ref mut function) = self.current_function {
+        if let Some(ref mut function) = self.scope_ctx.current_function {
             if let Some(block_data) = function.get_block_mut(block) {
                 // Find PHI instruction with matching dst
                 for inst in &mut block_data.instructions {
@@ -1076,7 +1068,7 @@ impl MirBuilder {
 
     /// Check if the current basic block is terminated
     fn is_current_block_terminated(&self) -> bool {
-        if let (Some(block_id), Some(ref function)) = (self.current_block, &self.current_function) {
+        if let (Some(block_id), Some(ref function)) = (self.current_block, &self.scope_ctx.current_function) {
             if let Some(block) = function.get_block(block_id) {
                 return block.is_terminated();
             }

src/mir/builder/control_flow/joinir/merge/loop_header_phi_builder.rs

@@ -223,7 +223,7 @@ impl LoopHeaderPhiBuilder {
 
         // Get the header block from current function
         let current_func = builder
-            .current_function
+            .scope_ctx.current_function
             .as_mut()
             .ok_or("Phase 33-16: No current function when finalizing header PHIs")?;
 

src/mir/builder/control_flow/joinir/routing.rs

@@ -28,7 +28,7 @@ impl MirBuilder {
     ) -> Result<Option<ValueId>, String> {
         // Get current function name
         let func_name = self
-            .current_function
+            .scope_ctx.current_function
             .as_ref()
             .map(|f| f.signature.name.clone())
             .unwrap_or_default();

src/mir/builder/if_form.rs

@@ -222,7 +222,7 @@ impl MirBuilder {
 
         // 関数名ガードチェック
         let func_name = self
-            .current_function
+            .scope_ctx.current_function
             .as_ref()
             .map(|f| f.signature.name.as_str())
             .unwrap_or("");

src/mir/builder/method_call_handlers.rs

@@ -24,7 +24,7 @@ impl MeCallPolicyBox {
     ) -> Result<Option<ValueId>, String> {
         // Instance box: prefer enclosing box method (lowered function) if存在
         let enclosing_cls: Option<String> = builder
-            .current_function
+            .scope_ctx.current_function
             .as_ref()
             .and_then(|f| f.signature.name.split('.').next().map(|s| s.to_string()));
 

src/mir/builder/observe/resolve.rs

@@ -25,7 +25,7 @@ fn sample_every() -> usize {
 /// Dev‑only: emit a resolve.try event（candidates inspection）。
 pub(crate) fn emit_try(builder: &MirBuilder, meta: serde_json::Value) {
     let fn_name = builder
-        .current_function
+        .scope_ctx.current_function
         .as_ref()
         .map(|f| f.signature.name.as_str());
     let region = builder.debug_current_region_id();
@@ -35,7 +35,7 @@ pub(crate) fn emit_try(builder: &MirBuilder, meta: serde_json::Value) {
 /// Dev‑only: emit a resolve.choose event（decision）。
 pub(crate) fn emit_choose(builder: &MirBuilder, meta: serde_json::Value) {
     let fn_name = builder
-        .current_function
+        .scope_ctx.current_function
         .as_ref()
         .map(|f| f.signature.name.as_str());
     let region = builder.debug_current_region_id();

src/mir/builder/observe/ssa.rs

@@ -35,7 +35,7 @@ pub(crate) fn emit_phi(builder: &MirBuilder, dst: ValueId, inputs: &Vec<(BasicBl
         "decided_origin": decided_o,
     });
     let fn_name = builder
-        .current_function
+        .scope_ctx.current_function
         .as_ref()
         .map(|f| f.signature.name.as_str());
     let region = builder.debug_current_region_id();

src/mir/builder/ops.rs

@@ -48,7 +48,7 @@ impl super::MirBuilder {
             BinaryOpType::Arithmetic(op) => {
                 // Dev: Lower '+' を演算子ボックス呼び出しに置換（既定OFF）
                 let in_add_op = self
-                    .current_function
+                    .scope_ctx.current_function
                     .as_ref()
                     .map(|f| f.signature.name.starts_with("AddOperator.apply/"))
                     .unwrap_or(false);
@@ -121,7 +121,7 @@ impl super::MirBuilder {
                     };
                     if !name.is_empty() {
                         let in_guard = self
-                            .current_function
+                            .scope_ctx.current_function
                             .as_ref()
                             .map(|f| f.signature.name.starts_with(guard_prefix))
                             .unwrap_or(false);
@@ -252,7 +252,7 @@ impl super::MirBuilder {
             BinaryOpType::Comparison(op) => {
                 // Dev: Lower 比較 を演算子ボックス呼び出しに置換（既定OFF）
                 let in_cmp_op = self
-                    .current_function
+                    .scope_ctx.current_function
                     .as_ref()
                     .map(|f| f.signature.name.starts_with("CompareOperator.apply/"))
                     .unwrap_or(false);
@@ -541,7 +541,7 @@ impl super::MirBuilder {
             };
             if !name.is_empty() {
                 let in_guard = self
-                    .current_function
+                    .scope_ctx.current_function
                     .as_ref()
                     .map(|f| f.signature.name.starts_with(guard_prefix))
                     .unwrap_or(false);

src/mir/builder/receiver.rs

@@ -23,7 +23,7 @@ pub fn finalize_method_receiver(builder: &mut MirBuilder, callee: &mut Callee) {
         // Optional dev trace for receiver aliases
         if std::env::var("NYASH_BUILDER_TRACE_RECV").ok().as_deref() == Some("1") {
             let current_fn = builder
-                .current_function
+                .scope_ctx.current_function
                 .as_ref()
                 .map(|f| f.signature.name.clone())
                 .unwrap_or_else(|| "<none>".to_string());

src/mir/builder/scope_context.rs

@@ -20,7 +20,7 @@ pub(in crate::mir::builder) use super::vars::lexical_scope::LexicalScopeFrame;
 
 /// Scope and control flow context for MIR building
 #[derive(Debug)]
-pub(super) struct ScopeContext {
+pub(in crate::mir) struct ScopeContext {
     // ---- Lexical scope management ----
     /// Stack of lexical scopes for block-scoped `local` declarations
     /// Tracks per-block shadowing so variables restore on scope exit
@@ -41,11 +41,11 @@ pub(super) struct ScopeContext {
 
     // ---- Function context ----
     /// Current function being built
-    pub(super) current_function: Option<MirFunction>,
+    pub(in crate::mir) current_function: Option<MirFunction>,
 
     /// Parameter names for current function
     /// Same lifecycle as current_function
-    pub(super) function_param_names: HashSet<String>,
+    pub(in crate::mir) function_param_names: HashSet<String>,
 
     // ---- Debug scope ----
     /// Stack of region identifiers (e.g., "loop#1/header", "join#3/join")

src/mir/builder/vars/lexical_scope.rs

@@ -35,27 +35,18 @@ impl Drop for LexicalScopeGuard {
 }
 
 impl super::super::MirBuilder {
-    #[allow(deprecated)]
     pub(in crate::mir::builder) fn push_lexical_scope(&mut self) {
-        // Phase 136 Step 3/7: Update both scope_ctx (SSOT) and legacy field (backward compat)
+        // Phase 2-4: Use scope_ctx only (legacy field removed)
         self.scope_ctx.push_lexical_scope();
-        self.lexical_scope_stack.push(LexicalScopeFrame::new());
     }
 
-    #[allow(deprecated)]
     pub(in crate::mir::builder) fn pop_lexical_scope(&mut self) {
-        // Phase 136 Step 3/7: Pop from scope_ctx (SSOT)
+        // Phase 2-4: Use scope_ctx only (legacy field removed)
         let frame = self
             .scope_ctx
             .pop_lexical_scope()
             .expect("COMPILER BUG: pop_lexical_scope without push_lexical_scope");
 
-        // Sync to legacy field
-        let _ = self
-            .lexical_scope_stack
-            .pop()
-            .expect("COMPILER BUG: legacy stack out of sync");
-
         // Restore ValueId mappings
         for (name, previous) in frame.restore {
             match previous {
@@ -82,13 +73,12 @@ impl super::super::MirBuilder {
         }
     }
 
-    #[allow(deprecated)]
     pub(in crate::mir::builder) fn declare_local_in_current_scope(
         &mut self,
         name: &str,
         value: ValueId,
     ) -> Result<(), String> {
-        // Phase 136 Step 3/7: Use scope_ctx (SSOT)
+        // Phase 2-4: Use scope_ctx (SSOT)
         let Some(frame) = self.scope_ctx.current_scope_mut() else {
             return Err("COMPILER BUG: local declaration outside lexical scope".to_string());
         };

src/mir/loop_api.rs

@@ -102,7 +102,7 @@ impl LoopBuilderApi for super::builder::MirBuilder {
     }
 
     fn add_predecessor(&mut self, block: BasicBlockId, pred: BasicBlockId) -> Result<(), String> {
-        if let Some(ref mut f) = self.current_function {
+        if let Some(ref mut f) = self.scope_ctx.current_function {
             if let Some(bb) = f.get_block_mut(block) {
                 bb.add_predecessor(pred);
                 Ok(())
@@ -115,7 +115,7 @@ impl LoopBuilderApi for super::builder::MirBuilder {
     }
 
     fn seal_block(&mut self, block: BasicBlockId) -> Result<(), String> {
-        if let Some(ref mut f) = self.current_function {
+        if let Some(ref mut f) = self.scope_ctx.current_function {
             if let Some(bb) = f.get_block_mut(block) {
                 bb.seal();
                 Ok(())
@@ -133,7 +133,7 @@ impl LoopBuilderApi for super::builder::MirBuilder {
         dst: ValueId,
         inputs: Vec<(BasicBlockId, ValueId)>,
     ) -> Result<(), String> {
-        if let Some(ref mut f) = self.current_function {
+        if let Some(ref mut f) = self.scope_ctx.current_function {
             crate::mir::ssot::cf_common::insert_phi_at_head_spanned(
                 f,
                 block,

src/mir/region/observer.rs

@@ -37,7 +37,7 @@ pub fn observe_control_form(builder: &mut MirBuilder, form: &ControlForm) {
     }
 
     let func_name = builder
-        .current_function
+        .scope_ctx.current_function
         .as_ref()
         .map(|f| f.signature.name.as_str())
         .unwrap_or("<unknown>");
@@ -94,7 +94,7 @@ pub fn observe_function_region(builder: &mut MirBuilder) {
     }
 
     let func_name = builder
-        .current_function
+        .scope_ctx.current_function
         .as_ref()
         .map(|f| f.signature.name.as_str())
         .unwrap_or("<unknown>");
@@ -107,7 +107,7 @@ pub fn observe_function_region(builder: &mut MirBuilder) {
     let id = RegionId(NEXT_REGION_ID.fetch_add(1, Ordering::Relaxed));
 
     let entry_block = builder
-        .current_function
+        .scope_ctx.current_function
         .as_ref()
         .map(|f| f.entry_block)
         .unwrap_or_else(|| crate::mir::BasicBlockId::new(0));

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,