feat: add FileStore tombstone, TTL & consumer state persistence (Task 2)

Port Go filestore tombstone/deletion tests, consumer state encode/decode,
consumer file store persistence, and message TTL enforcement. Adds
ConsumerStateCodec and ConsumerFileStore implementations.

17 new tests ported from filestore_test.go.

src/NATS.Server/JetStream/Storage/MsgBlock.cs

@@ -26,6 +26,9 @@ public sealed class MsgBlock : IDisposable
     private readonly SafeFileHandle _handle;
     private readonly Dictionary<ulong, (long Offset, int Length)> _index = new();
     private readonly HashSet<ulong> _deleted = new();
+    // Go: SkipMsg writes tombstone records with empty subject — tracked separately so
+    // recovery can distinguish intentional sequence gaps from soft-deleted messages.
+    private readonly HashSet<ulong> _skipSequences = new();
     private readonly long _maxBytes;
     private readonly ReaderWriterLockSlim _lock = new();
     private long _writeOffset;   // Tracks the append position independently of FileStream.Position
@@ -402,6 +405,7 @@ public sealed class MsgBlock : IDisposable
 
             _index[sequence] = (offset, encoded.Length);
             _deleted.Add(sequence);
+            _skipSequences.Add(sequence); // Track skip sequences separately for recovery
             // Note: intentionally NOT added to _cache since it is deleted.
 
             if (_totalWritten == 0)
@@ -447,6 +451,22 @@ public sealed class MsgBlock : IDisposable
         finally { _lock.ExitReadLock(); }
     }
 
+    /// <summary>
+    /// Returns the maximum skip-sequence written into this block (0 if none).
+    /// Skip sequences are intentional tombstones from SkipMsg/SkipMsgs —
+    /// they bump _last without storing a live message, so recovery must account
+    /// for them when computing the high-water mark.
+    /// </summary>
+    public ulong MaxSkipSequence
+    {
+        get
+        {
+            _lock.EnterReadLock();
+            try { return _skipSequences.Count > 0 ? _skipSequences.Max() : 0UL; }
+            finally { _lock.ExitReadLock(); }
+        }
+    }
+
     /// <summary>
     /// Exposes the set of soft-deleted sequence numbers for read-only inspection.
     /// Reference: golang/nats-server/server/filestore.go — dmap access for state queries.
@@ -582,7 +602,12 @@ public sealed class MsgBlock : IDisposable
             _index[record.Sequence] = (offset, recordLength);
 
             if (record.Deleted)
+            {
                 _deleted.Add(record.Sequence);
+                // Empty subject = skip/tombstone record (from SkipMsg/SkipMsgs).
+                if (string.IsNullOrEmpty(record.Subject))
+                    _skipSequences.Add(record.Sequence);
+            }
 
             if (count == 0)
                 _firstSequence = record.Sequence;