overhaul thread model and backup reliability

Thread model:
  - replace the IOState CAS state machine with a StampedLock: region IO
    (RegionFileStorage.write/flush, now both guarded via @WrapMethod) holds
    the read lock, backups hold the write lock; the old witness-restore
    pattern could wedge the state permanently
  - run saveEverything on the main thread (was called from the backup
    executor), with shutdown-aware waiting; skip autoSave/saveEverything
    while a backup holds the write lock so the main thread never blocks
  - broadcast player messages from the main thread, honor `broadcast`

  Backup correctness:
  - watcher reported the parent directory instead of the changed file, so
    every incremental backup was empty; change sets are now deduplicated,
    atomically drained and merged back on failure; OVERFLOW forces a full
    backup; watch roots are re-registered when their inode changes (world
    format conversion orphaned the watches)
  - fix the region reassembly pipeline: zlib was "decompressed" with
    DeflaterInputStream, writeChunk dropped the 5-byte chunk header,
    the length field was off by one, coordinates were transposed, and
    sector padding was wrong on 4096-byte multiples — every reassembled
    chunk in old bundles is unrecoverable garbage
  - full backups now collect the file list after saving, off-thread

  New features:
  - reflink (FICLONE) staging via FFM: on CoW filesystems the write lock
    is only held while cloning, packing/uploading runs unlocked
  - chained restore: restoring an incremental automatically applies its
    full base and every incremental in between, in order
  - generation-based rotation implementing keepBackups; whole generations
    are deleted so an incremental never loses its base
  - Backup records carry subject/createdAt/incremental for chaining

  Reliability:
  - atomic index persistence (tmp + ATOMIC_MOVE), ConcurrentHashMap for
    the tracked-backup index, fsync bundles before storing
  - BundleReader: overlay extraction for chains, zip-slip guard, fix
    reversed createSymbolicLink args, EOF instead of infinite loop on
    truncated bundles
  - LocalBackupStrategy.deleteBackup compared the wrong field and could
    never delete; S3 retries now catch SdkException (retry was dead code),
    drop ifNoneMatch for idempotent retries
  - ordered shutdown: SERVER_STOPPING awaits in-flight backups before the
    final world save; SERVER_STOPPED runs restore, closes storage and
    cleans the temp dir with isolated error handling
  - fix /backup suspend being registered under /backup create

  Tests: region round-trip byte-comparison, BundleReader overlay/zip-slip/
  symlink/truncation, backup chain resolution and rotation.

  Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main/java/io/ib67/sfcraft/Backup.java

@@ -2,17 +2,41 @@ package io.ib67.sfcraft;
 
 import net.minecraft.network.chat.Component;
 
+import java.time.Instant;
+import java.time.ZoneId;
+import java.time.format.DateTimeFormatter;
+
+/**
+ * @param subject   name of the backup subject this bundle belongs to; null on entries created
+ *                  by older versions (those are treated as standalone full backups)
+ * @param createdAt epoch millis of creation; 0 on legacy entries
+ */
 public record Backup(
         String name,
         String backupKey,
         String type,
         String from,
+        String subject,
         boolean incremental,
+        long createdAt,
         long sizeTotal
 ) {
-    public Component toText(){
-        return Component.nullToEmpty("sizeTotal: "+Math.round(((sizeTotal / 1024.0) / 1024.0) * 100) * 0.01 + "MiB, "+
-                (incremental ? "incremental" :"") + ", type: "+type+"."
-        );
+    private static final DateTimeFormatter TIME_FORMAT =
+            DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss").withZone(ZoneId.systemDefault());
+
+    public boolean legacy() {
+        return subject == null || createdAt <= 0;
+    }
+
+    public Component toText() {
+        var size = Math.round(((sizeTotal / 1024.0) / 1024.0) * 100) * 0.01;
+        var sb = new StringBuilder()
+                .append(incremental ? "INCR" : "FULL")
+                .append(", ").append(size).append("MiB")
+                .append(", type: ").append(type);
+        if (createdAt > 0) {
+            sb.append(", at ").append(TIME_FORMAT.format(Instant.ofEpochMilli(createdAt)));
+        }
+        return Component.nullToEmpty(sb.toString());
     }
 }