perf: reuse diff image buffer in Zone::CheckAlarms

Replace delete/new Image cycle with lazy-alloc + Assign(). When the
buffer already exists (every frame after the first), Assign() detects
matching dimensions and does a plain memcpy into the existing
allocation, eliminating an aligned malloc+free of ~2 MB per zone per
analyzed frame.

With 4 zones at 15 fps this removes 60 alloc/free cycles per second
from the analysis hot path. The HighlightEdges code path (analysis
images) still allocates a new Image and deletes the old diff buffer,
which is correct — the next Assign() will reallocate once to restore
the single-channel format, then resume reuse.

Behaviorally equivalent: Zone dimensions are constant during zone
lifetime, the destructor already handles cleanup via delete image,
and the only external consumer (Monitor::Analyse → AlarmImage →
Overlay) reads the image without storing pointers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/zm_zone.cpp

@@ -201,10 +201,14 @@ bool Zone::CheckAlarms(const Image *delta_image) {
     return false;
   }
 
-  if (image)
-    delete image;
-  // Get the difference image
-  Image *diff_image = image = new Image(*delta_image);
+  // Reuse diff image buffer — dimensions are constant per zone, so Assign()
+  // just does a memcpy into the existing allocation instead of free+malloc.
+  if (!image) {
+    image = new Image(*delta_image);
+  } else {
+    image->Assign(*delta_image);
+  }
+  Image *diff_image = image;
   int diff_width = diff_image->Width();
   uint8_t* diff_buff = diff_image->Buffer();
   uint8_t* pdiff;