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/mir/join_ir/mod.rs

@@ -30,6 +30,9 @@ pub mod lowering;
 // Phase 29 L-5.2: Progress verification
 pub mod verify;
 
+// Phase 30.x: JSON serialization (jsonir v0)
+pub mod json;
+
 // Re-export lowering functions for backward compatibility
 pub use lowering::{
     lower_funcscanner_trim_to_joinir, lower_min_loop_to_joinir, lower_skip_ws_to_joinir,