Key-based party roster, show party member names in battle UI

- party.jsonc's "members" is now a key/value object (e.g. "hero",
  "companion") instead of an array - partyInit seeds party_t.keys
  alongside members, and partyGetMemberIndexByKey/partyGetMemberKey do the
  lookups. Cutscene items (PARTY_ADD_MEMBER/PARTY_REMOVE_MEMBER/
  PARTY_HEAL_MEMBER) now take a "key" JSON field, resolved to a roster
  index at load time, instead of a raw index.
- Battle UI shows who's acting: uibattlemenu.c draws the current fighter's
  party key above the action menu, and uibattlehud.c draws each ally
  fighter's key as an extra line above their HP/MP.

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

test/save/savetestfixture.c

@@ -26,10 +26,10 @@ static char_t SAVE_TEST_FIXTURE_REAL_HOME[PATH_MAX];
 static bool_t SAVE_TEST_FIXTURE_HAD_REAL_HOME;
 
 static const char_t *SAVE_TEST_FIXTURE_PARTY_JSON =
-  "{\"members\":[{"
+  "{\"members\":{\"hero\":{"
     "\"attack\":10,\"defense\":5,\"magic\":0,\"speed\":10,\"luck\":0,"
     "\"healthMax\":30,\"mpMax\":10"
-  "}]}";
+  "}}}";
 
 static yyjson_doc *SAVE_TEST_FIXTURE_PARTY_DOC = NULL;