refactor(phi): Phase 57 PHI code reduction

## Changes

### 57-2-alt: Remove redundant ConservativeMerge call
- phi.rs called ConservativeMerge::analyze twice (once directly,
  once via merge_all_vars)
- Now merge_all_vars returns changed_vars, eliminating redundancy

### 57-3: Delete PhiMergeOps trait (dead code, 17 lines)
- PhiMergeOps trait was defined but never called
- Only impl in loop_builder.rs was unused
- PhiBuilderOps has replaced it

### 57-4: Add responsibility comments to infer_type_from_phi
- Document that it's the "last resort" for type inference
- Specify deletion conditions (JoinIR type annotations)

## Cumulative PHI reduction
- Phase 38: 90 lines
- Phase 40-4.1: 35 lines
- Phase 41-1: 99 lines
- Phase 47: 33 lines
- Phase 57: 17 lines (PhiMergeOps) + optimization
- Total: 365+ lines removed

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

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

src/mir/builder/phi_merge.rs

@@ -155,14 +155,19 @@ impl<'a> PhiMergeHelper<'a> {
     /// * `skip_var` - Optional variable name to skip (already merged elsewhere)
     ///
     /// # Returns
-    /// Ok(()) on success, Err(String) on failure
+    /// Ok(changed_vars) - Set of variables that were changed, for pin handling
+    ///
+    /// # Phase 57 改善
+    ///
+    /// 戻り値を `()` から `HashSet<String>` に変更。
+    /// これにより `phi.rs` での冗長な `ConservativeMerge::analyze` 呼び出しを削除可能に。
     pub fn merge_all_vars(
         &mut self,
         pre_if_snapshot: &BTreeMap<String, ValueId>, // Phase 25.1: BTreeMap化
         then_map_end: &BTreeMap<String, ValueId>,    // Phase 25.1: BTreeMap化
         else_map_end_opt: &Option<BTreeMap<String, ValueId>>, // Phase 25.1: BTreeMap化
         skip_var: Option<&str>,
-    ) -> Result<(), String> {
+    ) -> Result<std::collections::HashSet<String>, String> {
         // Use Conservative strategy from conservative module
         let conservative = crate::mir::phi_core::conservative::ConservativeMerge::analyze(
             pre_if_snapshot,
@@ -232,6 +237,7 @@ impl<'a> PhiMergeHelper<'a> {
             self.merge_variable(name.clone(), then_v, else_v)?;
         }
 
-        Ok(())
+        // Phase 57: 変更された変数セットを返す（phi.rsでの冗長呼び出し削除用）
+        Ok(conservative.changed_vars)
     }
 }