fix(player): a stale session answer can no longer stop the playback that won

- The engine tracks the request out per item: A, then B, then back to A sent a second request
  for A beside the one still out.
- A hardware transcode superseded during its fallback answered with the NEWER session; the losing
  request's player then stopped it as stale. It now answers with its own (dead) session.
- A replaced hls.js instance's late manifest no longer marks the player ready or plays the
  outgoing media.
- A pause given after the answer stands: the manifest and the first canplay read the live intent.
- The debounced A/V-offset restart is dropped on an item change instead of restarting the
  previous item under the next one's title.

backend/app/plex/stream.py

@@ -840,12 +840,13 @@ def start_session(
         )
         respawned = _spawn(False, False, replace_only=s)
         if respawned is None:
-            # Superseded/culled mid-fallback — yield whoever is registered now (a newer seek-restart), or
-            # our own now-dead session if the key was cap-culled. Never None here: the POST /session route
-            # turns a None return into a "Media file not found on disk" 404 — dishonest for a file that
-            # exists. A (dead) session instead degrades to a retryable 404 ("No active playback session" /
-            # "Not available yet") the client re-requests.
-            return current_session(user_id, key) or s
+            # Superseded/culled mid-fallback — answer with our own now-dead session. Never the newer one
+            # registered meanwhile: this answer goes to the request that LOST, whose player drops it as
+            # stale and stops the session it names (`stop_session`), which stopped the playback that won.
+            # Never None either: the POST /session route turns a None return into a "Media file not found
+            # on disk" 404 — dishonest for a file that exists. A dead session instead degrades to a
+            # retryable 404 ("No active playback session" / "Not available yet") the client re-requests.
+            return s
         s = respawned
         ready = wait_for(_seg(0), timeout=25.0, proc=s.proc)
     if ready:

backend/tests/test_plex_stream.py

@@ -695,7 +695,9 @@ class TestTranscodeStartSession:
         # F2/race: start_session waits on seg_0 OUTSIDE _lock. If a concurrent same-key start (a rapid
         # seek-restart) or _enforce_cap replaces _sessions[key] while we wait, our proc dying must NOT
         # trigger the SW fallback — that would tear down the NEWER session and breach the transcode cap.
-        # The identity guard must detect it's been superseded and yield the currently-registered session.
+        # The identity guard must detect it's been superseded. It answers with its OWN (dead) session,
+        # never the newer one: the answer goes to the request that lost, whose player drops it as stale
+        # and stops the session it names — naming the newer one stopped the playback that won.
         monkeypatch.setattr(
             stream, "_select_encode", lambda db, item, transcode, src: (True, False)
         )
@@ -708,9 +710,12 @@ class TestTranscodeStartSession:
 
         newer = _Newer()
 
+        captured = {}
+
         def wait_then_supersede(path, timeout=20.0, proc=None):
             # Simulate the concurrent replacement happening during our out-of-lock wait, then report the
             # (now-killed) HW proc produced nothing.
+            captured["s"] = stream._sessions[(7, "12345")]
             stream._sessions[(7, "12345")] = newer
             return False
 
@@ -720,7 +725,7 @@ class TestTranscodeStartSession:
         )
         # No SW respawn — only the initial HW _spawn ran; the newer session was left intact.
         assert len(_CapturingProc.calls) == 1
-        assert s is newer  # yielded the currently-registered session, did not clobber it
+        assert s is captured["s"]  # its own session, not the newer one
         assert stream._sessions[(7, "12345")] is newer
 
     def test_a_cap_culled_hw_session_does_not_respawn_and_is_not_reported_missing(