design(joinir): Phase 73 - ScopeManager BindingId Migration Design + PoC

Phase 73 plans migration from name-based to BindingId-based scope management
in JoinIR lowering, aligning with MIR's lexical scope model.

Design decision: Option A (Parallel BindingId Layer) with gradual migration.
Migration roadmap: Phases 74-77, ~8-12 hours total, zero production impact.

Changes:
- phase73-scope-manager-design.md: SSOT design (~700 lines)
- phase73-completion-summary.md: Deliverables summary
- phase73-index.md: Navigation index
- scope_manager_bindingid_poc/: Working PoC (437 lines, dev-only)

Tests: 6/6 PoC tests PASS, lib 950/950 PASS
Implementation: Parallel layer (no changes to existing code paths)

🤖 Generated with Claude Code

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

src/mir/join_ir/lowering/mod.rs

@@ -64,6 +64,8 @@ pub mod method_call_lowerer; // Phase 224-B: MethodCall lowering (metadata-drive
 pub(crate) mod step_schedule; // Phase 47-A: Generic step scheduler for P2/P3 (renamed from pattern2_step_schedule)
 pub mod method_return_hint; // Phase 83: P3-D 既知メソッド戻り値型推論箱
 pub mod scope_manager; // Phase 231: Unified variable scope management // Phase 195: Pattern 4 minimal lowerer
+#[cfg(feature = "normalized_dev")]
+pub mod scope_manager_bindingid_poc; // Phase 73: BindingId-based scope PoC (dev-only)
                        // Phase 242-EX-A: loop_with_if_phi_minimal removed - replaced by loop_with_if_phi_if_sum
 pub mod loop_with_if_phi_if_sum; // Phase 213: Pattern 3 AST-based if-sum lowerer (Phase 242-EX-A: supports complex conditions)
 pub mod min_loop;