refactor(phi_core): Phase 30 F-2.2 delete LoopSnapshotManager (test-only)

Remove LoopSnapshotManager which was only used by its own internal tests.
No external call sites existed.

- Delete loop_snapshot_manager.rs (~458 lines)
- Update mod.rs to remove module declaration
- Update PHI_BOX_INVENTORY.md and TASKS.md

Analysis showed all other F-2.2 candidates have production dependencies
and must wait for JoinIR coverage expansion:
- PhiBuilderBox - loop_builder.rs
- LoopSnapshotMergeBox - loopform_builder.rs, json_v0_bridge
- if_phi.rs - lifecycle.rs, if_form.rs, loop_builder.rs
- if_body_local_merge.rs - phi_builder_box.rs, loop_builder.rs
- phi_invariants.rs, conservative.rs, phi_input_collector.rs - multiple

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

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

src/mir/phi_core/mod.rs

@@ -24,7 +24,7 @@ pub mod phi_input_collector;
 
 // Phase 26-C: Loop Snapshot Management
 // Phase 30 F-2.1: header_phi_builder 削除（JoinIR loop_step で代替）
-pub mod loop_snapshot_manager;
+// Phase 30 F-2.2: loop_snapshot_manager 削除（テスト専用、外部呼び出しなし）
 
 // Phase 26-D: Exit PHI Management
 // Phase 30 F-2.1: exit_phi_builder 削除（JoinIR k_exit で代替、バイパス関数は loopform_builder に移動）