v0.4.18: fix /videoCache clear vs in-flight download race

clearAll() previously only wiped finalized files + the IN_FLIGHT set; any
download that was already running on the single-thread executor kept
writing past the clear, then atomically moved its .part to final and
re-published into READY — resurrecting one cache entry post-clear.

Fix: add an AtomicLong CACHE_EPOCH. preload() captures the epoch at submit
time; clearAll() bumps the epoch BEFORE wiping disk; download(url, epoch)
checks at three points (pre-start, in read loop, pre-publish) and bails if
the epoch moved, deleting its own .part only AFTER closing the output
stream (Windows can't delete an open file).

Phase ordering in clearAll() also matters: bump epoch first → drop
indexes → delete files. That way the in-flight download sees the
cancellation flag before the index wipe / file delete races can interact
with it.

gradle.properties

@@ -5,7 +5,7 @@ org.gradle.configuration-cache=false
 
 # Mod
 mod_id=video_player
-mod_version=0.4.17
+mod_version=0.4.18
 maven_group=com.ejclaw.videoplayer
 archives_base_name=video_player