feat(joinir): Phase 41-4.1 Add NestedIfMerge JoinInst variant

Add structural support for nested if patterns with PHI-sensitive variables:

1. JoinInst::NestedIfMerge in mod.rs
   - conds: Vec<VarId> (outer to inner conditions)
   - merges: Vec<MergePair> (variable updates at deepest level)
   - k_next: Option<JoinContId> (continuation after merge)

2. JSON serialization in json.rs
   - Type: "nested_if_merge"
   - Fields: conds[], merges[], k_next

3. Runner/Bridge stubs
   - JoinIR Runner: Returns error (use VM Bridge instead)
   - VM Bridge: panic placeholder (41-4.3 will implement)

Target: ParserControlBox.parse_loop() (4-level nested if)

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

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

src/mir/join_ir_runner.rs

@@ -190,6 +190,14 @@ fn execute_function(
                         "MethodCall is not supported in JoinIR Runner (use JoinIR→MIR→VM bridge instead)"
                     ));
                 }
+                // Phase 41-4: NestedIfMerge instruction execution
+                JoinInst::NestedIfMerge { .. } => {
+                    // Phase 41-4: NestedIfMerge は JoinIR Runner では未対応
+                    // JoinIR → MIR 変換経由で VM が実行する
+                    return Err(JoinRuntimeError::new(
+                        "NestedIfMerge is not supported in JoinIR Runner (use JoinIR→MIR→VM bridge instead)"
+                    ));
+                }
             }
         }