fix(raft): address WAL code quality issues — CRC perf, DRY, safety, assertions

- SyncAsync: remove redundant FlushAsync, use single Flush(flushToDisk:true)
- ComputeCrc: use incremental Crc32.Append to avoid contiguous buffer heap allocation
- Load: cast pos+length to long to guard against int overflow in bounds check
- AppendAsync: delegate to WriteEntryTo (DRY — eliminates duplicated record-building logic)
- Load: extract ParseEntries static helper to eliminate goto pattern with early returns
- Entries: change return type from IEnumerable to IReadOnlyList for index access and Count property
- RaftNode.PersistAsync: remove redundant term.txt write (meta.json now owns term+votedFor)
- RaftWalTests: tighten ShouldBeGreaterThanOrEqualTo(1) -> ShouldBe(1) in truncation/CRC tests;
  use .Count property directly on IReadOnlyList instead of .Count() LINQ extension

src/NATS.Server/Raft/RaftNode.cs

@@ -610,7 +610,6 @@ public sealed class RaftNode : IDisposable
         var dir = _persistDirectory ?? Path.Combine(Path.GetTempPath(), "natsdotnet-raft", Id);
         Directory.CreateDirectory(dir);
         await Log.PersistAsync(Path.Combine(dir, "log.json"), ct);
-        await File.WriteAllTextAsync(Path.Combine(dir, "term.txt"), TermState.CurrentTerm.ToString(), ct);
         await File.WriteAllTextAsync(Path.Combine(dir, "applied.txt"), AppliedIndex.ToString(), ct);
 
         // Persist term and VotedFor together in meta.json for atomic durable state.