media_controller: cancel stale prefetch spirals on new click

Each prefetch_adjacent() call now bumps a generation counter.
Running spirals check the counter at each iteration and exit
when superseded. Previously, rapid clicks between posts stacked
up concurrent download loops that never cancelled, accumulating
HTTP connections and response buffers.

Also incrementally updates the search controller's cached-names
set when a download completes, avoiding a full directory rescan.

behavior change: only the most recent click's prefetch spiral
runs; older ones exit at their next iteration.

booru_viewer/gui/media_controller.py

@@ -73,6 +73,7 @@ class MediaController:
         self._prefetch_pause = asyncio.Event()
         self._prefetch_pause.set()  # not paused
         self._last_evict_check = 0.0  # monotonic timestamp
+        self._prefetch_gen = 0  # incremented on each prefetch_adjacent call
 
     # -- Post activation (media load) --
 
@@ -162,6 +163,13 @@ class MediaController:
         idx = self._app._grid.selected_index
         if 0 <= idx < len(self._app._grid._thumbs):
             self._app._grid._thumbs[idx]._cached_path = path
+        # Keep the search controller's cached-names set current so
+        # subsequent _drain_append_queue calls see newly downloaded files
+        # without a full directory rescan.
+        cn = self._app._search_ctrl._cached_names
+        if cn is not None:
+            from pathlib import Path as _P
+            cn.add(_P(path).name)
         self._app._popout_ctrl.update_media(path, info)
         self.auto_evict_cache()
 
@@ -207,7 +215,12 @@ class MediaController:
             self._app._grid._thumbs[index].set_prefetch_progress(progress)
 
     def prefetch_adjacent(self, index: int) -> None:
-        """Prefetch posts around the given index."""
+        """Prefetch posts around the given index.
+
+        Bumps a generation counter so any previously running spiral
+        exits at its next iteration instead of continuing to download
+        stale adjacencies.
+        """
         total = len(self._app._posts)
         if total == 0:
             return
@@ -215,9 +228,16 @@ class MediaController:
         mode = self._app._db.get_setting("prefetch_mode")
         order = compute_prefetch_order(index, total, cols, mode)
 
+        self._prefetch_gen += 1
+        gen = self._prefetch_gen
+
         async def _prefetch_spiral():
             for adj in order:
+                if self._prefetch_gen != gen:
+                    return  # superseded by a newer prefetch
                 await self._prefetch_pause.wait()
+                if self._prefetch_gen != gen:
+                    return
                 if 0 <= adj < len(self._app._posts) and self._app._posts[adj].file_url:
                     self._app._signals.prefetch_progress.emit(adj, 0.0)
                     try:
@@ -251,7 +271,7 @@ class MediaController:
             for fav in self._app._db.get_bookmarks(limit=999999):
                 if fav.cached_path:
                     protected.add(fav.cached_path)
-            evicted = evict_oldest(max_bytes, protected)
+            evicted = evict_oldest(max_bytes, protected, current_bytes=current)
             if evicted:
                 log.info(f"Auto-evicted {evicted} cached files")
         max_thumb_mb = self._app._db.get_setting_int("max_thumb_cache_mb") or 500