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]>

assets/party.jsonc

@@ -1,13 +1,16 @@
 {
   // Starting party roster, loaded by partyInit into a fresh save slot.
-  "members": [
-    {
+  // Each key is a stable identifier cutscene content can reference
+  // directly (see PARTY_ADD_MEMBER/PARTY_REMOVE_MEMBER/PARTY_HEAL_MEMBER's
+  // "key" field) instead of a fragile positional index.
+  "members": {
+    "hero": {
       "attack": 10, "defense": 5, "magic": 0, "speed": 10, "luck": 0,
       "healthMax": 30, "mpMax": 10
     },
-    {
+    "companion": {
       "attack": 8, "defense": 6, "magic": 4, "speed": 8, "luck": 1,
       "healthMax": 25, "mpMax": 15
     }
-  ]
+  }
 }