feat(joinir): Phase 30.x jsonir v0 - JoinIR JSON serialization

Implement JSON serialization for JoinIR module.

Implementation:
- src/mir/join_ir/json.rs: JSON serializer (~250 lines, no external deps)
- src/tests/joinir_json_min.rs: Integration tests (8 unit + 2 integration)
- 10 tests total, all passing

Features:
- JoinModule → JSON serialization
- All instruction types: Call, Jump, Ret, Compute
- All MirLikeInst types: Const, BinOp, Compare, BoxCall
- Full ConstValue support: Integer, Bool, String, Null
- Full operator coverage: Add/Sub/Mul/Div/Or/And, Lt/Le/Gt/Ge/Eq/Ne
- JSON string escaping for special characters

Usage:
  use crate::mir::join_ir::json::join_module_to_json_string;
  let json = join_module_to_json_string(&module);

Non-goals (this phase):
- CLI flag (--emit-joinir-json)
- JSON → JoinIR reverse conversion

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

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

src/tests/mod.rs

@@ -6,6 +6,7 @@ pub mod identical_exec;
 pub mod identical_exec_collections;
 pub mod identical_exec_instance;
 pub mod identical_exec_string;
+pub mod joinir_json_min; // Phase 30.x: JoinIR JSON シリアライズテスト
 pub mod joinir_runner_min; // Phase 27.2: JoinIR 実行器 A/B 比較テスト
 pub mod joinir_runner_standalone; // Phase 27-shortterm S-3.2: JoinIR Runner 単体テスト
 pub mod joinir_vm_bridge_skip_ws; // Phase 27-shortterm S-4.4: JoinIR → Rust VM Bridge A/B Test