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/phi_core/if_phi.rs

@@ -136,7 +136,8 @@ pub fn infer_type_from_phi(
 ) -> Option<MirType> {
     for (_bid, bb) in function.blocks.iter() {
         for inst in bb.instructions.iter() {
-            if let MirInstruction::Phi { dst, inputs } = inst {
+            if let MirInstruction::Phi { dst, inputs, .. } = inst {
+                // Phase 63-6: type_hint is ignored here, will be used in lifecycle.rs
                 if *dst == ret_val {
                     let mut it = inputs.iter().filter_map(|(_, v)| types.get(v));
                     if let Some(first) = it.next() {