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.debug_scope_stack.push(region);
+        self.scope_ctx.debug_push_region(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();
             }