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.rs

@@ -21,19 +21,16 @@ impl MirBuilder {
         skip_var: Option<&str>,
     ) -> Result<(), String> {
         // 📦 Phase 25.1q: Use PhiMergeHelper for unified PHI insertion
+        // 📦 Phase 57: ConservativeMerge 冗長呼び出し削除
+        //    - 以前: ここで ConservativeMerge::analyze を呼び、merge_all_vars 内でも呼んでいた（2回）
+        //    - 現在: merge_all_vars が changed_vars を返すので、1回で済む
         use std::collections::HashSet;
 
-        let conservative = crate::mir::phi_core::conservative::ConservativeMerge::analyze(
-            pre_if_snapshot,
-            then_map_end,
-            else_map_end_opt,
-        );
-        let changed_set: HashSet<String> = conservative.changed_vars.iter().cloned().collect();
-
         // Use PhiMergeHelper for unified variable merging
         let mut helper =
             super::phi_merge::PhiMergeHelper::new(self, then_exit_block_opt, else_exit_block_opt);
-        helper.merge_all_vars(pre_if_snapshot, then_map_end, else_map_end_opt, skip_var)?;
+        let changed_set: HashSet<String> =
+            helper.merge_all_vars(pre_if_snapshot, then_map_end, else_map_end_opt, skip_var)?;
 
         // Ensure pinned synthetic slots ("__pin$...") have a block-local definition at the merge,
         // even if their values did not change across branches. This avoids undefined uses when