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/builder/control_flow.rs

@@ -542,7 +542,7 @@ impl super::MirBuilder {
                     values.insert(*v);
                 }
             }
-            MirInstruction::Phi { dst, inputs } => {
+            MirInstruction::Phi { dst, inputs, type_hint: None } => {
                 values.insert(*dst);
                 for (_, val) in inputs {
                     values.insert(*val);
@@ -651,12 +651,13 @@ impl super::MirBuilder {
             MirInstruction::Return { value } => MirInstruction::Return {
                 value: value.map(remap_value),
             },
-            MirInstruction::Phi { dst, inputs } => MirInstruction::Phi {
+            MirInstruction::Phi { dst, inputs, type_hint: None } => MirInstruction::Phi {
                 dst: remap_value(*dst),
                 inputs: inputs
                     .iter()
                     .map(|(bb, val)| (remap_block(*bb), remap_value(*val)))
                     .collect(),
+                type_hint: None, // Phase 63-6: Preserve no type hint during remapping
             },
             MirInstruction::Copy { dst, src } => MirInstruction::Copy {
                 dst: remap_value(*dst),