feat(startup): reduce time-to-playback from ~5s to ~300ms

Adopt the JS/CLI player's quick-start strategy: begin playback after
just 2 time sync measurements instead of waiting for full Kalman filter
convergence (5+ measurements).

Changes:
- Add HasMinimalSync property to IClockSynchronizer (true after 2 measurements)
- Use HasMinimalSync instead of IsConverged for playback gating
- Reduce convergence timeout from 5000ms to 1000ms as safety net

The sync correction system (resampling + drop/insert) handles any
initial estimation errors, making the long convergence wait unnecessary.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: chrisuthe

src/Sendspin.SDK/Audio/AudioPipeline.cs

@@ -426,8 +426,9 @@ private bool ShouldWaitForClockSync()
             return false;
         }
 
-        // If clock is already converged, proceed
-        if (_clockSync.IsConverged)
+        // If clock has minimal sync (2+ measurements), proceed
+        // Full convergence happens in background, sync correction handles any estimation errors
+        if (_clockSync.HasMinimalSync)
         {
             return false;
         }

src/Sendspin.SDK/Synchronization/KalmanClockSynchronizer.cs

@@ -43,6 +43,7 @@ public sealed class KalmanClockSynchronizer : IClockSynchronizer
 
     // Convergence tracking
     private const int MinMeasurementsForConvergence = 5;
+    private const int MinMeasurementsForPlayback = 2;  // Quick start: 2 measurements like JS/CLI players
     private const double MaxOffsetUncertaintyForConvergence = 1000.0; // 1ms uncertainty threshold
 
     // Drift reliability threshold - don't apply drift until uncertainty is below this
@@ -88,6 +89,7 @@ public int MeasurementCount
 
     /// <summary>
     /// Whether the synchronizer has converged to a stable estimate.
+    /// Requires 5+ measurements and low offset uncertainty.
     /// </summary>
     public bool IsConverged
     {
@@ -101,6 +103,26 @@ public bool IsConverged
         }
     }
 
+    /// <summary>
+    /// Whether the synchronizer has enough measurements for playback (at least 2).
+    /// Unlike <see cref="IsConverged"/>, this doesn't require statistical convergence.
+    /// The sync correction system handles any estimation errors during initial playback.
+    /// </summary>
+    /// <remarks>
+    /// This matches the JS/CLI player behavior which starts after 2 measurements (~300-500ms)
+    /// rather than waiting for full Kalman filter convergence (5+ measurements, ~1-5 seconds).
+    /// </remarks>
+    public bool HasMinimalSync
+    {
+        get
+        {
+            lock (_lock)
+            {
+                return _measurementCount >= MinMeasurementsForPlayback;
+            }
+        }
+    }
+
     /// <summary>
     /// Whether the drift estimate is reliable enough to use for time conversions.
     /// Drift estimation requires longer time periods to be accurate, so we don't
@@ -455,9 +477,16 @@ public interface IClockSynchronizer
 
     /// <summary>
     /// Whether the synchronizer has converged to a stable estimate.
+    /// Requires 5+ measurements and low offset uncertainty.
     /// </summary>
     bool IsConverged { get; }
 
+    /// <summary>
+    /// Whether the synchronizer has enough measurements for playback (at least 2).
+    /// Unlike <see cref="IsConverged"/>, this doesn't require statistical convergence.
+    /// </summary>
+    bool HasMinimalSync { get; }
+
     /// <summary>
     /// Resets the synchronizer state.
     /// </summary>

src/SendspinClient/appsettings.json

@@ -19,7 +19,7 @@
       "AdaptiveCutoff": 0.75,
       "MinSamplesForForgetting": 100,
       "WaitForConvergence": true,
-      "ConvergenceTimeoutMs": 5000
+      "ConvergenceTimeoutMs": 1000
     },
     "SyncCorrection": {
       "UseResampling": true,