Make party and save-slot init/new-game operate on SAVE.slot directly

party.c's functions no longer take a party_t* - they always operate on
SAVE.slot.party, since gameplay only ever has one active party. That
forced the same change on saveSlotInit/saveSlotNewGame (they called
partyInit/partyAddMember internally), so those are now parameterless
too, always resetting/seeding SAVE.slot. Callers building a save at a
specific index (new-game creation, slot deletion) now set
SAVE.slotCurrent first instead of building an independent local struct -
safe since the select-save screen is only ever reached before any
gameplay starts.

saveSlotWriteJSON/saveSlotReadJSON deliberately keep their explicit
saveslot_t* parameter - they're genuine serialization primitives used
by the save-device layer and tests against arbitrary structs, unrelated
to the single-current-party assumption. saveSlotReadJSON's internal
reset is now a plain memset instead of delegating to the now-global-only
saveSlotInit().

Updates test_save/test_savedevice/test_savedevicelinux/test_saveslot for
the new signatures. Note: test_savedevice/test_savedevicelinux/test_save
have pre-existing failures in this sandbox unrelated to this change -
savetestfixture.c swaps $HOME, but saveDeviceLinuxGetDirectory actually
derives the save path from ASSET.baseDirectory (the executable's own
location), so the fixture's sandboxing never actually applies. test_
saveslot.c (which doesn't depend on that fixture) passes 15/15.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

test/save/test_savedevicelinux.c

@@ -11,6 +11,7 @@
 #include "save/savedevicelinux.h"
 #include "save/slot/saveslot.h"
 #include "save/settings/savesettings.h"
+#include "save/save.h"
 #include "util/memory.h"
 #include "util/string.h"
 #include <stdio.h>
@@ -180,8 +181,9 @@ static void test_checkAvailability_mkdirpFails(void **state) {
 
 static void test_slotWrite_nullAsserts(void **state) {
   savedevice_t device;
+  // Content is never touched - both calls assert on a NULL argument
+  // before either device or slot would actually be read.
   saveslot_t slot;
-  saveSlotInit(&slot);
 
   expect_assert_failure(saveDeviceLinuxSlotWrite(NULL, &slot, 0));
   expect_assert_failure(saveDeviceLinuxSlotWrite(&device, NULL, 0));
@@ -190,7 +192,6 @@ static void test_slotWrite_nullAsserts(void **state) {
 static void test_slotRead_nullAsserts(void **state) {
   savedevice_t device;
   saveslot_t slot;
-  saveSlotInit(&slot);
 
   expect_assert_failure(saveDeviceLinuxSlotRead(NULL, &slot, 0));
   expect_assert_failure(saveDeviceLinuxSlotRead(&device, NULL, 0));
@@ -203,17 +204,21 @@ static void test_slotWriteRead_roundTrip(void **state) {
   // saveDeviceLinuxCheckAvailability does this via mkdirp.
   checkAvailabilityAndDrain(&device);
 
-  saveslot_t written;
-  saveSlotInit(&written);
-  stringCopy(written.cachedData.name, "Hero", sizeof(written.cachedData.name));
-  written.cachedData.playerLevel = 9;
+  // saveSlotInit() only ever operates on SAVE.slot now, so that's the
+  // write source here.
+  saveSlotInit();
+  stringCopy(
+    SAVE.slot.cachedData.name, "Hero", sizeof(SAVE.slot.cachedData.name)
+  );
+  SAVE.slot.cachedData.playerLevel = 9;
 
-  errorret_t writeRet = saveDeviceLinuxSlotWrite(&device, &written, 1);
+  errorret_t writeRet = saveDeviceLinuxSlotWrite(&device, &SAVE.slot, 1);
   assert_true(errorIsOk(writeRet));
 
+  // No pre-init needed - saveSlotReadJSON() (via saveDeviceLinuxSlotRead)
+  // fully resets/overwrites its destination struct regardless of prior
+  // content.
   saveslot_t read;
-  saveSlotInit(&read);
-
   errorret_t readRet = saveDeviceLinuxSlotRead(&device, &read, 1);
   assert_true(errorIsOk(readRet));
 
@@ -225,8 +230,11 @@ static void test_slotRead_noFileYet_leavesSlotUntouched(void **state) {
   savedevice_t device;
   memoryZero(&device, sizeof(device));
 
+  // Deliberately not saveSlotInit()'d - a missing slot file means
+  // saveDeviceLinuxSlotRead never even calls saveSlotReadJSON, so slot
+  // must come in exactly as the caller left it and go out unchanged.
   saveslot_t slot;
-  saveSlotInit(&slot);
+  memoryZero(&slot, sizeof(slot));
   stringCopy(slot.cachedData.name, "sentinel", sizeof(slot.cachedData.name));
   slot.cachedData.playerLevel = 555;
 
@@ -260,7 +268,6 @@ static void test_slotRead_corruptFile_errors(void **state) {
   fclose(file);
 
   saveslot_t slot;
-  saveSlotInit(&slot);
 
   errorret_t ret = saveDeviceLinuxSlotRead(&device, &slot, 0);
   assert_true(errorIsNotOk(ret));