perf: Phase 2 lazy StoredMessage materialization in FileStore

Replace eager Dictionary<ulong, StoredMessage> with lightweight
Dictionary<ulong, MessageMeta> to eliminate ~200B StoredMessage
allocation per message on the write path.

- Add MessageMeta struct (BlockId, Subject, PayloadLength, HeaderLength,
  TimestampNs) — ~40B vs ~200B for StoredMessage
- Add MaterializeMessage(seq) for on-demand reconstruction from blocks
- Update all ~60 _messages references to use _meta
- Methods needing full payload (LoadAsync, ListAsync, etc.) call
  MaterializeMessage; metadata-only paths use _meta directly
- Fix MsgBlock.WriteAt to clear stale delete markers on re-write

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

@@ -315,6 +315,10 @@ public sealed class MsgBlock : IDisposable
 
             _index[sequence] = (offset, written);
 
+            // If this sequence was previously soft-deleted, clear the deletion marker
+            // so that subsequent Read calls return the new record rather than null.
+            _deleted.Remove(sequence);
+
             // Go: cache populated lazily on read, not eagerly on write.
             // Reads that miss _cache flush pending buf to disk and decode from there.