refactor(mir): Phase 25.1n - レガシーコード削除（未使用コード整理）

**削減内容**:
- loop_builder.rs: incomplete_phis, emit_safepoint, mark_block_sealed (-28行)
- builder.rs: hint_loop_*, debug_loop_*, debug_replace_region (-28行)
- builder/loops.rs: create_loop_blocks (-9行)

**成果**:
- コード削減: 65行の未使用コード削除
- 警告削減: 7個 → 2個 (71%削減)
- 機能維持: すべてのテスト通過 ✅

**削除されたレガシーAPI**:
- incomplete_phis フィールド (LoopFormBuilder移行後未使用)
- emit_safepoint() メソッド (未使用)
- mark_block_sealed() メソッド (未使用)
- hint_loop_header/latch/carrier() (ヒントシステム未使用)
- debug_next_loop_id() (デバッグ機能未使用)
- debug_replace_region() (デバッグ機能未使用)
- create_loop_blocks() (LoopForm v2移行後未使用)

🧹 箱化・モジュール化の第一歩として未使用コード整理完了

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/builder.rs

@@ -186,8 +186,7 @@ pub struct MirBuilder {
     // ----------------------
     /// Stack of region identifiers like "loop#1/header" or "join#3/join".
     debug_scope_stack: Vec<String>,
-    /// Monotonic counters for region IDs (deterministic across a run).
-    debug_loop_counter: u32,
+    /// Monotonic counter for region IDs (deterministic across a run).
     debug_join_counter: u32,
 
     /// Local SSA cache: ensure per-block materialization for critical operands (e.g., recv)
@@ -268,7 +267,6 @@ impl MirBuilder {
 
             // Debug scope context
             debug_scope_stack: Vec::new(),
-            debug_loop_counter: 0,
             debug_join_counter: 0,
 
             local_ssa_map: HashMap::new(),
@@ -290,30 +288,15 @@ impl MirBuilder {
 
     // ---- Hint helpers (no-op by default) ----
     #[inline]
-    pub(crate) fn hint_loop_header(&mut self) { self.hint_sink.loop_header(); }
-    #[inline]
-    pub(crate) fn hint_loop_latch(&mut self) { self.hint_sink.loop_latch(); }
-    #[inline]
     pub(crate) fn hint_scope_enter(&mut self, id: u32) { self.hint_sink.scope_enter(id); }
     #[inline]
     pub(crate) fn hint_scope_leave(&mut self, id: u32) { self.hint_sink.scope_leave(id); }
     #[inline]
     pub(crate) fn hint_join_result<S: Into<String>>(&mut self, var: S) { self.hint_sink.join_result(var.into()); }
-    #[inline]
-    pub(crate) fn hint_loop_carrier<S: Into<String>>(&mut self, vars: impl IntoIterator<Item = S>) {
-        self.hint_sink.loop_carrier(vars.into_iter().map(|s| s.into()).collect::<Vec<_>>());
-    }
 
     // ----------------------
     // Debug scope helpers (region_id for DebugHub events)
     // ----------------------
-    #[inline]
-    pub(crate) fn debug_next_loop_id(&mut self) -> u32 {
-        let id = self.debug_loop_counter;
-        self.debug_loop_counter = self.debug_loop_counter.saturating_add(1);
-        id
-    }
-
     #[inline]
     pub(crate) fn debug_next_join_id(&mut self) -> u32 {
         let id = self.debug_join_counter;
@@ -331,15 +314,6 @@ impl MirBuilder {
         let _ = self.debug_scope_stack.pop();
     }
 
-    #[inline]
-    pub(crate) fn debug_replace_region<S: Into<String>>(&mut self, region: S) {
-        if let Some(top) = self.debug_scope_stack.last_mut() {
-            *top = region.into();
-        } else {
-            self.debug_scope_stack.push(region.into());
-        }
-    }
-
     #[inline]
     pub(crate) fn debug_current_region_id(&self) -> Option<String> {
         self.debug_scope_stack.last().cloned()