StaticImageRenderer: fix mip selection off-by-one, narrow draw lock (#177)

- SelectBitmapForScale: clamp max was _mipChain.Length-1, meaning the
  finest mip level was never reachable and single-level chains always
  fell back to the source bitmap; fix to _mipChain.Length
- Draw: narrow _mipChainLock to cover only SelectBitmapForScale; moving
  DrawImage outside the lock avoids holding it across the D2D command
  recording window
- KickOffMipGeneration: discard Task.Run result explicitly (_ =) to
  make fire-and-forget intent clear to the compiler

Author: riyasy

Src/FlyPhotos/Display/ImageRendering/StaticImageRenderer.cs

@@ -55,7 +55,7 @@ private void KickOffMipGeneration()
     {
         _mipGenCts = new CancellationTokenSource();
         var token = _mipGenCts.Token;
-        Task.Run(() => GenerateMipChainAsync(token), token);
+        _ = Task.Run(() => GenerateMipChainAsync(token), token);
     }
 
     private async Task GenerateMipChainAsync(CancellationToken token)
@@ -142,11 +142,10 @@ public void Draw(CanvasDrawingSession session, CanvasViewState viewState, Canvas
         }
 
 
+        CanvasBitmap src;
         lock (_mipChainLock)
-        {
-            var src = SelectBitmapForScale(viewState.Scale);
-            session.DrawImage(src, viewState.ImageRect, src.Bounds, 1f, quality);
-        }
+            src = SelectBitmapForScale(viewState.Scale);
+        session.DrawImage(src, viewState.ImageRect, src.Bounds, 1f, quality);
 
     }
 
@@ -161,7 +160,7 @@ private CanvasBitmap SelectBitmapForScale(float scale)
 
         // k = floor(log2(1/scale)): 0 at scale≥1, 1 at scale<0.5, 2 at scale<0.25, …
         var k = (int)Math.Floor(Math.Log2(1.0 / scale));
-        k = Math.Clamp(k, 0, _mipChain.Length - 1);
+        k = Math.Clamp(k, 0, _mipChain.Length);
         return k == 0 ? _sourceBitmap : _mipChain[k - 1];
     }