feat(joinir): Phase 63-4 infer_type_from_phi degradation design

Phase 63-4: infer_type_from_phi を『JoinIR 型ヒント優先＋従来ロジックフォールバック』に縮退する仕様を設計（実装は Phase 63-5+）

## Changes

### Documentation Updates
- **README.md**: Added complete Phase 63-4 design (63-4.1 through 63-4.5)
  - 63-4.1: Current state analysis (definition location, callsites, role, JoinIR preparation)
  - 63-4.2: Degradation spec (type_hint priority + fallback pattern)
  - 63-4.3: Representative cases and A/B testing strategy (P1/P2/P3)
  - 63-4.4: Deletion conditions (5 conditions, current: 2/5 = 40%)
  - 63-4.5: Phase 63-5 handoff (infer_type_from_phi_with_hint() implementation tasks)

- **PHI_BOX_INVENTORY.md**: Updated if_phi.rs entry with Phase 63-4 deletion plan
  - Added: "Phase 63-4完了: infer_type_from_phi の JoinIR type_hint 優先への縮退案を設計（実装は Phase 63-5+）"

- **CURRENT_TASK.md**: Added Phase 63-4 section with summary of design work

## Design Highlights

### Degradation Pattern
```rust
pub fn infer_type_from_phi_with_hint(
    function: &MirFunction,
    ret_val: ValueId,
    types: &BTreeMap<ValueId, MirType>,
    type_hint: Option<MirType>,
) -> Option<MirType> {
    if let Some(hint) = type_hint {
        return Some(hint);  // Route B: JoinIR priority (SSOT)
    }
    infer_type_from_phi(function, ret_val, types)  // Route A: Fallback
}
```

### Representative Cases
- **P1**: IfSelectTest.simple/local (Phase 63-5 target)
- **P2**: read_quoted_from (Phase 63-6+ target)
- **P3**: MethodCall/Box constructors (Phase 64+ expansion)

### Deletion Conditions (2/5 achieved)
1. ✅ JoinIR has type_hint field (Phase 63-3)
2. ✅ Type hints populated for representative cases (Phase 63-2)
3. ⏳ Degraded to type_hint priority (Phase 63-5)
4. ⏳ P1 cases determined by type_hint only (Phase 63-5)
5. ⏳ All functions use type hints (Phase 64+)

## Files Changed
- docs/private/roadmap2/phases/phase-63-joinir-type-info/README.md
- docs/private/roadmap2/phases/phase-30-final-joinir-world/PHI_BOX_INVENTORY.md
- CURRENT_TASK.md

## Next Steps
Phase 63-5: Implement degradation for P1 cases (IfSelectTest.simple/local)

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

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

src/mir/join_ir/mod.rs

@@ -23,6 +23,8 @@
 use std::collections::BTreeMap;
 
 use crate::mir::ValueId;
+// Phase 63-3: 型ヒント用
+use crate::mir::MirType;
 
 // Phase 27.9: Lowering submodule
 pub mod lowering;
@@ -206,6 +208,8 @@ pub struct MergePair {
     pub then_val: VarId,
     /// else 分岐での値
     pub else_val: VarId,
+    /// Phase 63-3: 結果型ヒント（MIR PHI 生成時の型推論を回避）
+    pub type_hint: Option<MirType>,
 }
 
 /// JoinIR 命令セット（最小版）
@@ -296,11 +300,15 @@ pub enum JoinInst {
 
     /// Phase 33: If/Else の単純な値選択（単一値）
     /// cond が true なら then_val、false なら else_val を dst に代入
+    ///
+    /// Phase 63-3: type_hint で結果型を伝播（infer_type_from_phi 削減用）
     Select {
         dst: VarId,
         cond: VarId,
         then_val: VarId,
         else_val: VarId,
+        /// Phase 63-3: 結果型ヒント（MIR PHI 生成時の型推論を回避）
+        type_hint: Option<MirType>,
     },
 
     /// Phase 33-6: If/Else の複数変数 merge