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/lowering/if_merge.rs

@@ -202,6 +202,7 @@ impl IfMergeLowerer {
                 dst,
                 then_val,
                 else_val,
+                type_hint: None, // Phase 63-3: 現時点では型ヒントなし
             });
         }
 

src/mir/join_ir/lowering/if_select.rs

@@ -12,6 +12,8 @@
 
 use crate::mir::join_ir::JoinInst;
 use crate::mir::{BasicBlockId, MirFunction, MirInstruction, ValueId};
+// Phase 63-2: Type hint inference from MIR
+use crate::mir::{ConstValue, MirType};
 
 // Phase 61-1: If-in-loop context support
 use super::if_phi_context::IfPhiContext;
@@ -118,11 +120,16 @@ impl IfSelectLowerer {
         // Select 命令を生成
         let dst = pattern.dst.unwrap_or(pattern.then_val);
 
+        // Phase 63-2: MIR の Const 命令から型ヒントを推論
+        let type_hint = infer_type_from_mir_pattern(func, pattern.then_val)
+            .or_else(|| infer_type_from_mir_pattern(func, pattern.else_val));
+
         Some(JoinInst::Select {
             dst,
             cond: pattern.cond,
             then_val: pattern.then_val,
             else_val: pattern.else_val,
+            type_hint, // Phase 63-2: Const 命令から推論した型（Integer/Bool/String など）
         })
     }
 
@@ -364,3 +371,29 @@ impl IfSelectLowerer {
         })
     }
 }
+
+/// Phase 63-2: MIR から ValueId の型を推論
+///
+/// Const 命令を探して、ValueId に対応する MirType を返す。
+/// Select の then_val / else_val から型ヒントを埋めるために使用。
+fn infer_type_from_mir_pattern(func: &MirFunction, val_id: ValueId) -> Option<MirType> {
+    // 全ブロックの全命令を走査して Const 命令を探す
+    for block in func.blocks.values() {
+        for inst in &block.instructions {
+            if let MirInstruction::Const { dst, value } = inst {
+                if *dst == val_id {
+                    return Some(match value {
+                        ConstValue::Integer(_) => MirType::Integer,
+                        ConstValue::Bool(_) => MirType::Bool,
+                        ConstValue::String(_) => MirType::String,
+                        ConstValue::Void => MirType::Void,
+                        ConstValue::Null => MirType::Unknown, // Null は Unknown として扱う
+                        // Float は現状未サポート
+                        _ => return None,
+                    });
+                }
+            }
+        }
+    }
+    None
+}