feat(mir): Phase 63-6-1/2 MIR Phi type_hint field & JoinIR propagation

Phase 63-6-1: MirInstruction::Phi に type_hint フィールド追加
- Added `type_hint: Option<MirType>` field to Phi instruction
- Updated 21 files with type_hint initialization (all set to None for legacy paths)
- Pattern matching updated across codebase (11 files)
- Test code updated (basic_block.rs)

Phase 63-6-2: JoinIR→MIR Bridge で型ヒント伝播実装
- Modified convert.rs: Select → MIR now creates PHI with type_hint
- Removed Copy instructions from then/else blocks
- PHI instruction at merge block receives type_hint from JoinIR Select
- Test verification: ✅ Type hint propagation successful (Some(Integer))

Modified files:
- instruction.rs: Added type_hint field definition
- join_ir_vm_bridge/convert.rs: Select lowering with PHI + type_hint
- 19 other files: type_hint field initialization

Test results:
- ✅ test_type_hint_propagation_simple: Type hint = Some(Integer) confirmed
- ✅ 7/8 if_select tests passing (1 race condition, passes individually)

Next: Phase 63-6-3 (lifecycle.rs で型ヒント使用)

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

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

src/mir/instruction.rs

@@ -129,9 +129,17 @@ pub enum MirInstruction {
     // === SSA Phi Function ===
     /// SSA phi function for merging values from different paths
     /// `%dst = phi [%val1 from %bb1, %val2 from %bb2, ...]`
+    ///
+    /// # Phase 63-6: Type Hint Support
+    ///
+    /// `type_hint` field stores type information from JoinIR (Select/IfMerge)
+    /// to enable type inference without scanning PHI inputs.
+    /// - `Some(MirType)`: Type is known from JoinIR (P1 cases: IfSelectTest.simple/local)
+    /// - `None`: Type must be inferred from PHI inputs (legacy behavior)
     Phi {
         dst: ValueId,
         inputs: Vec<(super::BasicBlockId, ValueId)>,
+        type_hint: Option<super::MirType>, // Phase 63-6: JoinIR type hint
     },
 
     // === Box Operations ===