Add versioned save slot/settings schemas with strict version checks

Splits saveslot_t/savesettings_t into a stable, version-independent
in-memory struct (save/slot/, save/settings/) and a separate frozen
per-schema-version struct (saveslotver1_t/savesettingsver1_t) used only
for JSON serialization, converted between manually. Every write now
stamps a "version" field (SAVE_SLOT_CURRENT/SAVE_SETTINGS_CURRENT), and
every read requires it to match exactly, rejecting anything else -
laying the groundwork for a real migration path instead of silently
misreading old/incompatible save data.

test/save/test_savedevicelinux.c

@@ -9,8 +9,8 @@
 #include "savetestfixture.h"
 #include "save/savedevice.h"
 #include "save/savedevicelinux.h"
-#include "save/saveslot.h"
-#include "save/savesettings.h"
+#include "save/slot/saveslot.h"
+#include "save/settings/savesettings.h"
 #include "util/memory.h"
 #include "util/string.h"
 #include <stdio.h>
@@ -298,7 +298,7 @@ static void test_settingsWriteRead_roundTrip(void **state) {
 
   savesettings_t written;
   saveSettingsInit(&written);
-  written.someSetting = 314;
+  written.audioMasterVolume = 3.25f;// exactly representable, safe to compare
 
   errorret_t writeRet = saveDeviceLinuxSettingsWrite(&device, &written);
   assert_true(errorIsOk(writeRet));
@@ -308,7 +308,7 @@ static void test_settingsWriteRead_roundTrip(void **state) {
 
   errorret_t readRet = saveDeviceLinuxSettingsRead(&device, &read);
   assert_true(errorIsOk(readRet));
-  assert_int_equal(read.someSetting, 314);
+  assert_true(read.audioMasterVolume == 3.25f);
 }
 
 static void test_settingsRead_noFileYet_leavesUntouched(void **state) {
@@ -317,11 +317,11 @@ static void test_settingsRead_noFileYet_leavesUntouched(void **state) {
 
   savesettings_t settings;
   saveSettingsInit(&settings);
-  settings.someSetting = 777;
+  settings.audioMasterVolume = 7.75f;
 
   errorret_t ret = saveDeviceLinuxSettingsRead(&device, &settings);
   assert_true(errorIsOk(ret));
-  assert_int_equal(settings.someSetting, 777);
+  assert_true(settings.audioMasterVolume == 7.75f);
 }
 
 static void test_settingsRead_corruptFile_errors(void **state) {