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

tests/NATS.Server.Tests/Raft/RaftWalTests.cs

@@ -37,7 +37,7 @@ public class RaftWalTests : IDisposable
 
         // Recover
         using var recovered = RaftWal.Load(walPath);
-        var entries = recovered.Entries.ToList();
+        var entries = recovered.Entries;
         entries.Count.ShouldBe(3);
         entries[0].Index.ShouldBe(1);
         entries[0].Term.ShouldBe(1);
@@ -60,7 +60,7 @@ public class RaftWalTests : IDisposable
         await wal.CompactAsync(5); // remove entries 1-5
 
         using var recovered = RaftWal.Load(walPath);
-        recovered.Entries.Count().ShouldBe(5);
+        recovered.Entries.Count.ShouldBe(5);
         recovered.Entries.First().Index.ShouldBe(6);
     }
 
@@ -82,7 +82,7 @@ public class RaftWalTests : IDisposable
             fs.SetLength(fs.Length - 3);
 
         using var recovered = RaftWal.Load(walPath);
-        recovered.Entries.Count().ShouldBeGreaterThanOrEqualTo(1);
+        recovered.Entries.Count.ShouldBe(1);
         recovered.Entries.First().Command.ShouldBe("good-entry");
     }
 
@@ -118,7 +118,7 @@ public class RaftWalTests : IDisposable
         }
 
         using var recovered = RaftWal.Load(walPath);
-        recovered.Entries.Count().ShouldBe(0);
+        recovered.Entries.Count.ShouldBe(0);
     }
 
     // Go reference: server/raft.go WAL CRC integrity check
@@ -139,9 +139,9 @@ public class RaftWalTests : IDisposable
         bytes[^5] ^= 0xFF;
         File.WriteAllBytes(walPath, bytes);
 
-        // Load should recover at least the first record, stopping at the corrupt second
+        // Load should recover exactly the first record, stopping at the corrupt second
         using var recovered = RaftWal.Load(walPath);
-        recovered.Entries.Count().ShouldBeGreaterThanOrEqualTo(1);
+        recovered.Entries.Count.ShouldBe(1);
         recovered.Entries.First().Command.ShouldBe("valid");
     }
 }