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/frontend/ast_lowerer/if_in_loop/lowering/single_var_both.rs

@@ -42,6 +42,7 @@ pub fn lower(
         cond: cond_id,
         then_val,
         else_val,
+        type_hint: None, // Phase 63-3
     });
 
     ctx.register_param(var_name.to_string(), result_id);

src/mir/join_ir/frontend/ast_lowerer/if_in_loop/lowering/single_var_then.rs

@@ -40,6 +40,7 @@ pub fn lower(
         cond: cond_id,
         then_val,
         else_val,
+        type_hint: None, // Phase 63-3
     });
 
     ctx.register_param(var_name.to_string(), result_id);

src/mir/join_ir/frontend/ast_lowerer/if_return.rs

@@ -128,6 +128,7 @@ impl AstToJoinIrLowerer {
             cond: cond_var,
             then_val: then_var,
             else_val: else_var,
+            type_hint: None, // Phase 63-3
         });
 
         // Ret result

src/mir/join_ir/frontend/ast_lowerer/loop_patterns/continue_pattern.rs

@@ -212,6 +212,7 @@ fn create_loop_step_function_continue(
         cond: continue_cond_var,
         then_val: step_acc,      // Continue: 更新しない
         else_val: acc_increment, // 通常: 更新
+        type_hint: None,         // Phase 63-3
     });
 
     // 6. 末尾再帰

src/mir/join_ir/frontend/ast_lowerer/loop_patterns_old.rs

@@ -843,6 +843,7 @@ impl AstToJoinIrLowerer {
             cond: continue_cond_var,
             then_val: step_acc,      // Continue: 更新しない
             else_val: acc_increment, // 通常: 更新
+            type_hint: None,         // Phase 63-3
         });
 
         // 6. 末尾再帰

src/mir/join_ir/frontend/ast_lowerer/nested_if.rs

@@ -145,6 +145,7 @@ impl AstToJoinIrLowerer {
                     dst,
                     then_val: then_var,
                     else_val: else_var,
+                    type_hint: None, // Phase 63-3
                 });
             }
 

src/mir/join_ir/frontend/ast_lowerer/read_quoted.rs

@@ -403,11 +403,13 @@ impl AstToJoinIrLowerer {
                         dst: i_after_esc,
                         then_val: i_esc,
                         else_val: step_i,
+                        type_hint: None, // Phase 63-3
                     },
                     MergePair {
                         dst: ch_merged,
                         then_val: ch_esc,
                         else_val: step_ch,
+                        type_hint: None, // Phase 63-3
                     },
                 ],
                 k_next: None,