docs(joinir): Phase 56 - Ownership-Relay Design + interface skeleton

「読むのは自由、管理は直下 owned だけ」アーキテクチャの設計文書と型定義。

Key changes:
- Design doc: phase56-ownership-relay-design.md
  - Core definitions: owned/carriers/captures/relay
  - Invariants: Ownership Uniqueness, Carrier Locality, Relay Propagation
  - Shadowing rules, multi-writer merge semantics
  - JoinIR mapping from current system to new system
  - Implementation phases roadmap (56-61)

- New module: src/mir/join_ir/ownership/
  - types.rs: ScopeId, ScopeOwnedVar, RelayVar, CapturedVar, OwnershipPlan
  - mod.rs: Module documentation with responsibility boundaries
  - README.md: Usage guide and examples

- API methods:
  - OwnershipPlan::carriers() - owned AND written variables
  - OwnershipPlan::condition_only_carriers() - condition-only carriers
  - OwnershipPlan::verify_invariants() - invariant checking

Tests: 942/942 PASS (+3 unit tests)
Zero behavioral change - analysis module skeleton only.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/mir/join_ir/mod.rs

@@ -41,6 +41,9 @@ pub mod normalized;
 // Phase 34-1: Frontend (AST→JoinIR) — skeleton only
 pub mod frontend;
 
+// Phase 56: Ownership analysis (reads/writes → owned/relay/capture)
+pub mod ownership;
+
 // 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,