Add player name field to save file, save it out as a round-trip test

Adds savefile_t.playerName (SAVE_PLAYER_NAME_MAX) serialized via the
existing saveFileReadString/WriteString helpers, and stamps + saves it in
rpgInit() as a test that the save system now persists actual game data,
not just the header/version. Verified manually: written bytes end in
"Dusk\0" immediately after the version field, and loading it back returns
the same string.

src/dusk/rpg/rpg.c

@@ -18,6 +18,8 @@
 #include "util/memory.h"
 #include "util/string.h"
 #include "assert/assert.h"
+#include "save/save.h"
+#include "error/error.h"
 
 #include "ui/rpg/uiemoji.h"
 
@@ -51,6 +53,12 @@ errorret_t rpgInit(void) {
   backpackAdd(ITEM_ID_POTATO, 3);
   backpackAdd(ITEM_ID_APPLE, 8);
 
+  // TEST: Verify the save system round-trips real game data, not just the
+  // header/version. Remove once there's an actual name-entry flow.
+  savefile_t *saveFile = saveGet(0);
+  stringCopy(saveFile->playerName, "Dusk", SAVE_PLAYER_NAME_MAX);
+  errorCatch(errorPrint(saveWrite(0)));
+
   // All Good!
   errorOk();
 }

src/dusk/save/savefile.h

@@ -20,6 +20,9 @@
 /** Maximum number of independent save slots supported. */
 #define SAVE_FILE_COUNT_MAX 3
 
+/** Maximum length of a saved player name, including the null terminator. */
+#define SAVE_PLAYER_NAME_MAX 32
+
 typedef struct {
   /** Magic header bytes read from the file; must equal SAVE_FILE_HEADER. */
   char_t header[SAVE_FILE_HEADER_SIZE];
@@ -27,4 +30,6 @@ typedef struct {
   uint32_t version;
   /** Runtime flag - true if this slot was successfully loaded or written. */
   bool_t exists;
+  /** The player's saved name. */
+  char_t playerName[SAVE_PLAYER_NAME_MAX];
 } savefile_t;

src/dusk/save/savestream.c

@@ -330,11 +330,13 @@ errorret_t saveStreamWriteDateImpl(
 errorret_t saveFileLoad(savestream_t *stream, savefile_t *file) {
   saveFileReadHeader(stream, file->header);
   saveFileReadVersion(stream, &file->version);
+  saveFileReadString(stream, file->playerName, SAVE_PLAYER_NAME_MAX);
   errorOk();
 }
 
 errorret_t saveFileWrite(savestream_t *stream, savefile_t *file) {
   saveFileWriteHeader(stream, file->header);
   saveFileWriteVersion(stream, &file->version);
+  saveFileWriteString(stream, file->playerName, SAVE_PLAYER_NAME_MAX);
   errorOk();
 }