refactor: phases 5 & 6 — TransferService, Result<T>, RAII

Phase 5 — TransferService extraction:
- New nxst::TransferService owns all sender/receiver state, threads, and atomics
  (previously 11 file-statics across transfer_sender.cpp + transfer_receiver.cpp)
- startReceive() calls io::restore internally; UI never touches infra/fs or infra/net
- TitlesLayout routes all transfer ops through mainApp->transfer
- g_currentUId removed from global scope; TitlesLayout::InitTitles(AccountUid)
- MainApplication gains transfer member; layout refs renamed to snake_case
- userAppExit() updated to cancel via service

Phase 6 — Result<T> + RAII:
- include/nxst/domain/result.hpp: 85-line tagged-union Result<T,E> + Result<void,E>
- include/nxst/infra/fs/handles.hpp: FsFileSystemHandle (auto fsFsClose),
  FileHandle (auto fclose) — eliminates manual close on every error path
- io::backup / io::restore return nxst::Result<std::string> (was tuple<bool,Result,string>)
- new u8[] + malloc in copyFile/restore replaced with std::vector<u8>
- NACP save-creation extracted to createSaveIfNeeded() helper in io.cpp

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

PLAN.md

@@ -9,13 +9,13 @@
 | 2 | File renames + `#pragma once` | ✅ Done | S (~2h) | snake_case filenames, unify guards |
 | 3 | Directory restructure | ✅ Done | M (~1d) | `src/` + `include/nxst/` layered tree |
 | 4 | Make → CMake migration | ✅ Done | M (~1d) | devkitpro `Switch.cmake` toolchain |
-| 5 | TransferService extraction | ☐ Not started | L (~2d) | kill globals, sever UI ↔ net coupling |
-| 6 | `Result<T>` + RAII | ☐ Not started | M (~1d) | tagged union, OS handle wrappers, split `restore()` |
+| 5 | TransferService extraction | ✅ Done | L (~2d) | kill globals, sever UI ↔ net coupling |
+| 6 | `Result<T>` + RAII | ✅ Done | M (~1d) | tagged union, OS handle wrappers, fix raw memory |
 | 7 | Documentation + license | ☐ Not started | S (~half-day) | README, ARCHITECTURE, PROTOCOL, CHANGELOG, GPLv3 LICENSE |
 | 8 | CI | ☐ Not started | S (~2h) | GitHub Actions, `.nro` artifact, format check, layering check |
 
-**Active phase:** Phase 5 — TransferService extraction.
-**Last updated:** 2026-04-26.
+**Active phase:** Phase 7 — Documentation + license.
+**Last updated:** 2026-04-27.
 
 Mark a phase `🟡 In progress` when starting and `✅ Done` when verified on hardware. Keep this table the source of truth.