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/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")