fix(player,reporting): report real positions, and count an audio-only episode as watched

Returning to the foreground before the background-audio stream had started
playing handed the frontend 0.0s, so the video reloaded at StartTimeTicks=0 —
the episode restarted from the beginning — and the stop report that followed
wrote that zero to Jellyfin as the resume point. Caught on device: locked at
18.4s, unlocked 3.5s later with ExoPlayer still IDLE.

The base that turns a handoff's relative timeline into the episode's is applied
once at the native tick boundary (DR-159), so before the first tick nothing has
applied it. The same blind spot covers webview-rendered media, where nothing is
loaded into the native backend at all and its position is a permanent 0 — which
is why 14 of 14 stop reports in a 35-minute trace were zeroes, one landing 40s
after the frontend had correctly reported 15:22 for the same episode.

- absolute_position(): the maximum of the backend's reading, the last position
  webview media reported, and the handoff base. Exact rather than heuristic —
  at most one term is ever meaningful, and the base is a floor the stream
  cannot physically be behind. duration() gains the same fallback.
- Withhold zero-position stop reports. A zero is never information, and
  Jellyfin stores the reported position as the resume point, so sending one
  only ever destroys a real one.
- Report progress from the controller's own position ticks, through the 30s
  throttler it already shared with the native audio path.
  /Sessions/Playing/Progress was previously requested zero times in 35 minutes.
- Report a finished audio-only episode stopped at its runtime before advancing,
  so Jellyfin's 90% rule marks it played. Nothing else can: the webview is
  suspended and its <video> was torn down at the handoff.
- Split the handoff by source — a downloaded file takes no base and a real
  seek, a stream keeps its StartTimeTicks base and no seek — and stop routing a
  downloaded handoff's absolute seek through the stream rebuild, which refuses
  a non-remote source outright.

Reports go through a PlaybackReportSink, which also collapses three copies of
spawn-a-task-and-hope into one and is what let each of these be written as a
failing test first.

TRACES: UR-005, UR-025, UR-040, UR-071 | DR-178, DR-179, DR-180 |
        UT-176, UT-177, UT-178, UT-179, UT-180, UT-181

src-tauri/src/player/stream_end.rs

@@ -144,6 +144,18 @@ impl ObservedTime {
         live.filter(|p| *p >= 0.0).unwrap_or(self.position)
     }
 
+    /// The last observed position, with no live reading to prefer — the case
+    /// where the *reporter* is the only source there is (webview-rendered media,
+    /// which the native backend cannot see at all).
+    pub fn last_position(&self) -> f64 {
+        self.position
+    }
+
+    /// The last observed duration, if one was ever established.
+    pub fn last_duration(&self) -> Option<f64> {
+        self.duration
+    }
+
     /// The live reading if there is one, else the last observed value.
     pub fn duration_or_last(&self, live: Option<f64>) -> Option<f64> {
         live.filter(|d| *d > 0.0).or(self.duration)