fix(orbit): hysteresis on Catch Up visibility so the button stays clickable (#527)

* fix(orbit): keep Catch Up button visible long enough to click

Reported during testing: the Catch Up button "appears for a moment,
then disappears too fast to click". The single-stage debounce only
filtered the show direction (require ≥ 3 s sustained over-threshold
before showing), but on a real-world high-latency session the genuine
drift fluctuates between ~1 s and ~8 s in lockstep with both sides'
chunked `currentTime` updates — so the button vanished as soon as
drift dipped briefly under 3 s, even though the drift baseline was
still 5–8 s.

Add a hide-side hysteresis: once visible, the button stays visible
until drift has been under a tighter 1 s threshold for ≥ 1 s.
Otherwise the noisy 1–3 s drift valleys keep the button up so the
user can actually click it.

Constants left as locals; if testing wants different floors we can
extract.

* docs(changelog): add catch-up hysteresis bullet

* fix(orbit): show Catch Up when host paused + serialise seek-then-pause

Two follow-ons after the hysteresis change:

**1. Show Catch Up even when the host is paused.** The visibility
   gate required `state.isPlaying === true`, but a guest who joined
   while the host was paused still benefits from Catch Up if their
   sync to the host's paused position failed (engine drop, brief
   network blip during initial-sync, manual local seek). Drop the
   `state.isPlaying` gate — the only signal that should matter is
   "drift between us and the host's reported state" — which
   `computeOrbitDriftMs` already computes correctly in the paused
   case (no time-extrapolation, just `guestPos - hostPos`).

**2. Defer the play-state mirror after `player.seek`.** Reported
   symptom: "I press Catch Up, the player pauses; when I press play,
   it resumes from the *old* position and the waveform jumps back."
   Root cause: `player.seek` debounces `audio_seek` via
   `setTimeout(0)`, while `player.pause` / `player.resume` fire their
   invokes synchronously. Pause arrives at the engine first, leaving
   it paused at the pre-Catch-Up position with the seek queued
   behind. When resume happens later, the engine plays from the old
   spot. Push the play-state mirror behind a 200 ms `setTimeout` so
   the seek's invoke lands first.

src/components/OrbitSessionBar.tsx

@@ -66,18 +66,37 @@ export default function OrbitSessionBar() {
     return () => window.clearInterval(id);
   }, [state, phase]);
 
-  // ── Catch Up button visibility — debounced ────────────────────────────
+  // ── Catch Up button visibility — debounced + hysteresis ───────────────
   // The raw drift signal is noisy: guest's `currentTime` updates in coarse
   // ~5 s chunks while host's position is extrapolated linearly via
-  // `(nowMs - posAt)`, so the diff swings ±5 s every tick on a normal
-  // session even when both sides are perfectly synced. Without debounce
-  // the button flickered in and out and (worse) shifted the bar height.
-  // Require ≥ 3 s of sustained over-threshold drift before showing.
+  // `(nowMs - posAt)`, so the diff swings between ~1 s and ~8 s every
+  // tick on a normal session even when both sides are perfectly synced.
+  // Two-stage filter:
+  //   - **Hidden → shown**: drift must stay over the show-threshold (3 s)
+  //     for 3 s of wall-clock. Filters out brief over-threshold blips.
+  //   - **Shown → hidden**: drift must stay under the hide-threshold
+  //     (1 s) for 1 s of wall-clock. Once visible, the button persists
+  //     through the 1–3 s "drift back to small" valleys that come from
+  //     guest's currentTime catching up in chunks; otherwise the button
+  //     would vanish too fast to actually click on a high-latency
+  //     session where genuine drift fluctuates around 5–8 s.
+  const SHOW_THRESHOLD_MS = CATCH_UP_DRIFT_THRESHOLD_MS;
+  const HIDE_THRESHOLD_MS = 1_000;
+  const SHOW_DEBOUNCE_MS = 3_000;
+  const HIDE_DEBOUNCE_MS = 1_000;
   const [showCatchUp, setShowCatchUp] = useState(false);
   const overSinceRef = useRef<number | null>(null);
+  const underSinceRef = useRef<number | null>(null);
   useEffect(() => {
-    if (role !== 'guest' || !state || !state.isPlaying || !state.currentTrack) {
+    // Note: `state.isPlaying` is *not* a gate. A guest who joined while
+    // the host was paused still benefits from Catch Up if their sync to
+    // the host's paused position failed — the only signal that matters
+    // is "is there drift between us and the host's last reported state".
+    // `computeOrbitDriftMs` correctly stops time-extrapolation when the
+    // host is paused, so the formula holds in both states.
+    if (role !== 'guest' || !state || !state.currentTrack) {
       overSinceRef.current = null;
+      underSinceRef.current = null;
       setShowCatchUp(false);
       return;
     }
@@ -86,15 +105,34 @@ export default function OrbitSessionBar() {
     const driftMs = player.currentTrack?.id === state.currentTrack.trackId
       ? computeOrbitDriftMs(state, localPositionMs, nowMs)
       : null;
-    const isOver = driftMs == null || Math.abs(driftMs) > CATCH_UP_DRIFT_THRESHOLD_MS;
-    if (isOver) {
-      if (overSinceRef.current === null) overSinceRef.current = Date.now();
-      if (Date.now() - overSinceRef.current >= 3000) setShowCatchUp(true);
-    } else {
+    const absDrift = driftMs == null ? Infinity : Math.abs(driftMs);
+    if (showCatchUp) {
+      // Currently visible — only hide once drift has been clearly small
+      // for the full hide-debounce window.
       overSinceRef.current = null;
-      setShowCatchUp(false);
+      if (absDrift < HIDE_THRESHOLD_MS) {
+        if (underSinceRef.current === null) underSinceRef.current = Date.now();
+        if (Date.now() - underSinceRef.current >= HIDE_DEBOUNCE_MS) {
+          setShowCatchUp(false);
+          underSinceRef.current = null;
+        }
+      } else {
+        underSinceRef.current = null;
+      }
+    } else {
+      // Currently hidden — only show after sustained over-threshold drift.
+      underSinceRef.current = null;
+      if (absDrift > SHOW_THRESHOLD_MS) {
+        if (overSinceRef.current === null) overSinceRef.current = Date.now();
+        if (Date.now() - overSinceRef.current >= SHOW_DEBOUNCE_MS) {
+          setShowCatchUp(true);
+          overSinceRef.current = null;
+        }
+      } else {
+        overSinceRef.current = null;
+      }
     }
-  }, [role, state, nowMs]);
+  }, [role, state, nowMs, showCatchUp]);
 
   // Bar is visible while active, ended (pre-ack), or explicitly kicked / soft-removed.
   const shouldShowBar = !!state && (
@@ -143,9 +181,24 @@ export default function OrbitSessionBar() {
       const player = usePlayerStore.getState();
       const fraction = targetSec / Math.max(1, track.duration);
       if (player.currentTrack?.id === trackId) {
+        // `player.seek` debounces the underlying `audio_seek` invoke via
+        // `setTimeout(0)`, while `pause`/`resume` fire their invokes
+        // synchronously. Calling them back-to-back races on the Tauri
+        // command queue: pause/resume can arrive at the engine before
+        // the seek does, leaving the engine paused at the *old*
+        // position with the seek queued behind it — when the user
+        // hits play later the engine resumes from the pre-Catch-Up
+        // spot and the waveform jumps back. Defer the play-state
+        // mirror by one short tick so the seek lands first.
         player.seek(fraction);
-        if (hostPlaying && !player.isPlaying) player.resume();
-        else if (!hostPlaying && player.isPlaying) player.pause();
+        if (hostPlaying !== player.isPlaying) {
+          window.setTimeout(() => {
+            const p = usePlayerStore.getState();
+            if (p.currentTrack?.id !== trackId) return;
+            if (hostPlaying && !p.isPlaying) p.resume();
+            else if (!hostPlaying && p.isPlaying) p.pause();
+          }, 200);
+        }
       } else {
         // Different track: play + seek once the engine reports the track
         // loaded. The previous 400 ms blind delay was too short for an