Add a real test suite for the save system (previously had none)

Covers save.c (device discovery/orchestration), savedevice.c (the
generic device state machine and platform dispatch), and the Linux
platform backend (path building, availability checks, JSON read/write,
corrupt/missing-file handling), plus saveslot.c/savesettings.c JSON
round-trips. 88 tests across 5 files, all run against the real Linux
filesystem backend sandboxed to a temp $HOME (there's no mockable
platform layer - the hooks are compile-time macros, not function
pointers).

Deliberately locks in two existing behaviors rather than working around
them: saveSaveSettings() is a permanent no-op because nothing anywhere
ever sets SAVE.settingsDirty = true, and saveUpdate() unconditionally
rewrites settings back out the moment a device is found regardless of
that same dirty flag. Both are pre-existing, not introduced here.

Does not cover the SAVE_DEVICE_DATA_RAW blob codec (PSP/GameCube/Wii
only) or GameCube's 2-device fallback chain - neither compiles into the
Linux host test build.

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

test/CMakeLists.txt

@@ -12,5 +12,6 @@ add_subdirectory(thread)
 add_subdirectory(display)
 add_subdirectory(rpg)
 add_subdirectory(item)
+add_subdirectory(save)
 add_subdirectory(time)
 add_subdirectory(util)