port to Minecraft 26.3

No source changes were needed — nothing JustBackup touches moved in 26.3.
Version bumps only:

- minecraft 26.2 -> 26.3, loader 0.19.3 -> 0.19.5
- fabric-api 0.154.2+26.2 -> 0.160.5+26.3
- fabric.mod.json depends: minecraft ~26.2 -> ~26.3,
  fabricloader >=0.19.3 -> >=0.19.5
- README version line

Verified with `./gradlew build` against 26.3 (clean).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -2,7 +2,7 @@
 
 A fast, space-efficient backup mod for Fabric servers, written by iceBear67.
 
-- **Minecraft 26.2** · Fabric Loader ≥ 0.19.3 · Java 25
+- **Minecraft 26.3** · Fabric Loader ≥ 0.19.5 · Java 25
 - Consistent backups: no torn region files, even while the server is running
 - Incremental backups with **chained restore** (a full base + its incrementals restore as one unit)
 - **Reflink (CoW) fast path**: on btrfs/XFS/ZFS the world is only "frozen" for milliseconds