fix(player): play-next must use insertedAt, not size-1, when idle

Final review of caeef65 caught a stale-currentIndex bug in
/play-next-song and cmdPlayNext: when the player is idle but
queue.currentIndex >= 0 (natural end-of-track, or playNext gave
up after retries without queue.clear), addNext splices mid-queue
and playAt(size-1) jumps PAST the inserted song to whatever
was last. Capture the insertion slot before calling addNext and
promote that exact index instead.

Add a regression test covering the [a,b,c,d] queue with
currentIndex=1 case -- splice at 2 yields x at index 2, but
size-1 would point at d (index 4).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/audio/queue.test.ts

@@ -459,5 +459,29 @@ describe("PlayQueue", () => {
       // prev again → pop 0 → song at index 0 = a
       expect(queue.prev()?.id).toBe("a");
     });
+
+    it("idle player + stale currentIndex: insertion target is currentIndex+1, not size-1", () => {
+      // Reproduces the scenario where the player has gone idle but the
+      // queue still has a non-negative currentIndex (e.g., after natural
+      // track end without queue.clear()).
+      queue.add(makeSong("a"));
+      queue.add(makeSong("b"));
+      queue.add(makeSong("c"));
+      queue.add(makeSong("d"));
+      queue.play();      // current = 0 (a)
+      queue.next();      // current = 1 (b)
+      // Simulate idle-with-stale-currentIndex: the player has gone idle
+      // but queue still points at b.
+      // Caller pre-captures insertedAt:
+      const insertedAt = queue.getCurrentIndex() + 1; // = 2
+      queue.addNext(makeSong("x"));
+      // queue is now [a, b, x, c, d]
+      // size-1 would be 4 (d) — WRONG.
+      // insertedAt is 2 (x) — RIGHT.
+      expect(queue.list().map((s) => s.id)).toEqual(["a", "b", "x", "c", "d"]);
+      expect(queue.size() - 1).toBe(4); // proves size-1 strategy would pick d
+      const promoted = queue.playAt(insertedAt);
+      expect(promoted?.id).toBe("x");
+    });
   });
 });

src/bot/instance.ts

@@ -442,11 +442,19 @@ export class BotInstance extends EventEmitter {
 
     const song = result.songs[0];
     const wasIdle = this.player.getState() === "idle";
+    // Capture the slot addNext WILL insert at, before mutating the queue.
+    // addNext pushes when currentIndex<0 (slot = size); otherwise splices
+    // at currentIndex+1. Using size-1 after addNext was wrong when the
+    // queue had stale currentIndex>=0 while the player was idle (e.g.,
+    // after natural track end without queue.clear()).
+    const insertedAt =
+      this.queue.getCurrentIndex() < 0
+        ? this.queue.size()
+        : this.queue.getCurrentIndex() + 1;
     this.queue.addNext({ ...song, platform: provider.platform });
 
     if (wasIdle) {
-      // Nothing playing — addNext fell through to push, promote and start.
-      this.queue.playAt(this.queue.size() - 1);
+      this.queue.playAt(insertedAt);
       this.player.resetFailures();
       const ok = await this.resolveAndPlay(this.queue.current()!);
       this.emit("stateChange");

src/web/api/player.ts

@@ -352,12 +352,18 @@ export function createPlayerRouter(
       }
       const queue = bot.getQueueManager();
       const wasIdle = bot.getPlayer().getState() === "idle";
+      // Capture the slot addNext WILL insert at, before mutating the queue.
+      // addNext pushes when currentIndex<0 (slot = size); otherwise splices
+      // at currentIndex+1. Using size-1 after addNext was wrong when the
+      // queue had stale currentIndex>=0 while the player was idle (e.g.,
+      // after natural track end without queue.clear()).
+      const insertedAt =
+        queue.getCurrentIndex() < 0 ? queue.size() : queue.getCurrentIndex() + 1;
       queue.addNext(song);
 
       if (wasIdle) {
-        // No current playback — promote the just-added song to current
-        // and start it. addNext fell through to push, so it's the last item.
-        queue.playAt(queue.size() - 1);
+        // Promote the just-added song to current and start it.
+        queue.playAt(insertedAt);
         bot.getPlayer().resetFailures();
         const ok = await bot.resolveAndPlay(queue.current()!);
         if (!ok) {