v0.4.16: fix v0.4.15 reviewer-flagged regressions

1. VideoPlayerConfig.save() now persists max_cache_mb. Previously the
   auto-augment path in load() called save(), but save() didn't emit the
   key — so the rewrite kept losing it. Now updated comment + property.

2. /videoCache clear sends a new ClearCachePayload (no body) that wipes
   the entire client video_player_cache/ directory. Previously it only
   broadcast a DeleteCachePayload per *registered* URL, so leftover files
   from legacy preload_urls or prior sessions were never cleaned; now
   we always send clearAll regardless of whether the named index was
   non-empty.

3. Downloads are now serialized through a single-thread executor instead
   of `new Thread(...).start()`. With parallel downloads, every worker
   was snapshotting cacheDirSize() before any .part was renamed to its
   final name, so 50 simultaneous joins could collectively bust the
   max_cache_mb cap. With one in-flight at a time, each download's
   directory scan reflects every preceding completion.

Network: new S2C ClearCachePayload(unit codec), registered in
VideoPlayerNetwork + ClientNetworking. VideoCache.clearAll() iterates
the dir, deletes regular files, resets READY/IN_FLIGHT, reports
deleted/failed counts in chat + log. /videoCache clear command always
broadcasts to all clients now.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

gradle.properties

@@ -5,7 +5,7 @@ org.gradle.configuration-cache=false
 
 # Mod
 mod_id=video_player
-mod_version=0.4.15
+mod_version=0.4.16
 maven_group=com.ejclaw.videoplayer
 archives_base_name=video_player