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/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,