Persist SaveDataMemory2 save data to disk (fixes Astro Bot progress not saving) (#659)

* Persist save-data memory across restarts

* Fix SetSaveDataMemory2 data loss, split saves per user, add atomic writes everywhere

- SaveDataSetSaveDataMemory2 treated data_num=0 as "no entries" even when
  a valid data pointer was supplied. Astro Bot calls this with data_num=0
  meaning "one entry, at data[0]" (an implicit-count convention), so every
  real save write from it was silently dropped as a no-op. Default
  data_num to 1 when data is non-null, matching that convention.
- SaveDataSetupSaveDataMemory2/Get/SetSaveDataMemory2 rejected any call
  that included a non-null icon pointer. Icon persistence isn't
  implemented, but titles are allowed to pass one anyway; rejecting the
  whole call over it made every title that does so fail outright. Ignore
  icon instead of rejecting.
- SaveDataMount3/SaveDataTransferringMount/SaveDataDelete built their
  save-data path from title+dir_name only, dropping the user_id PS5
  local users are keyed by (unlike the newer SetSaveDataMemory2 layout,
  which already nests by user_id). Nest these under user_id too so saves
  from different local users can no longer collide. No migration for the
  old flat layout - the project is still pre-release/experimental.
- Classic SaveDataMount3-mounted titles (Blasphemous, Dead Cells, ...)
  write their save files directly through the guest's normal file I/O
  once mounted, bypassing SaveData's own write path entirely, so they
  had none of the crash-safety SetSaveDataMemory2 already had. Added the
  same protection at the file-system layer: a guest file opened under a
  /savedataN mount that recreates the file from scratch (O_CREAT and/or
  O_TRUNC) is now written to a .tmp shadow file and atomically replaces
  the real one on close, instead of being truncated in place.
- Both write paths now back up the file they're about to replace into a
  sibling "backups" folder (same filename, no added suffix) before the
  atomic replace, so a manual recovery copy of the previous save always
  survives a write - even one that "succeeds" but wasn't wanted.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Fix save-memory compatibility and preserve existing saves

---------

Co-authored-by: nmzik <Nmzik@mail.ru>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

CMakeLists.txt

@@ -450,6 +450,12 @@ add_executable(audio_out2_port_tests EXCLUDE_FROM_ALL
 target_link_libraries(audio_out2_port_tests common fmt::fmt)
 target_include_directories(audio_out2_port_tests PRIVATE ${inc_headers})
 
+add_executable(save_data_memory_tests EXCLUDE_FROM_ALL
+	"${KYTY_TESTS_DIR}/SaveDataMemoryTests.cpp"
+)
+target_link_libraries(save_data_memory_tests common fmt::fmt)
+target_include_directories(save_data_memory_tests PRIVATE ${inc_headers})
+
 add_executable(event_queue_lifetime_tests EXCLUDE_FROM_ALL
 	"${KYTY_TESTS_DIR}/EventQueueLifetimeTests.cpp"
 	"${KYTY_SOURCE_DIR}/kernel/eventQueue.cpp"
@@ -534,6 +540,7 @@ if(BUILD_TESTING)
 	add_test(NAME kernel_file_system COMMAND $<TARGET_FILE:kernel_file_system_tests>)
 	add_test(NAME sync_on_address COMMAND $<TARGET_FILE:sync_on_address_tests>)
 	add_test(NAME audio_out2_port COMMAND $<TARGET_FILE:audio_out2_port_tests>)
+	add_test(NAME save_data_memory COMMAND $<TARGET_FILE:save_data_memory_tests>)
 	add_test(NAME shader_recompiler_compute COMMAND $<TARGET_FILE:shader_recompiler_compute_tests>)
 	add_test(NAME shader_recompiler_alignbyte
 		COMMAND $<TARGET_FILE:shader_recompiler_compute_tests> --alignbyte-only)