feat(storage): rewrite FileStore to use block-based MsgBlock storage

Replace JSONL persistence with real MsgBlock-based block files (.blk).
FileStore now acts as a block manager that creates, seals, and rotates
MsgBlocks while maintaining an in-memory cache for fast reads/queries.

Key changes:
- AppendAsync writes transformed payloads to MsgBlock via WriteAt
- Block rotation occurs when active block reaches size limit
- Recovery scans .blk files and rebuilds in-memory state from records
- Legacy JSONL migration: existing messages.jsonl data is automatically
  converted to block files on first open, then JSONL is deleted
- PurgeAsync disposes and deletes all block files
- RewriteBlocks rebuilds blocks from cache (used by trim/restore)
- InvalidDataException propagates during recovery (wrong encryption key)

MsgBlock.WriteAt added to support explicit sequence numbers and timestamps,
needed when rewriting blocks with non-contiguous sequences (after removes).

Tests updated:
- New FileStoreBlockTests.cs with 9 tests for block-specific behavior
- JetStreamFileStoreCompressionEncryptionParityTests updated to read
  FSV1 magic from .blk files instead of messages.jsonl
- JetStreamFileStoreDurabilityParityTests updated to verify .blk files
  instead of index.manifest.json

All 3,562 tests pass (3,535 passed + 27 skipped, 0 failures).

tests/NATS.Server.Tests/JetStream/JetStreamFileStoreCompressionEncryptionParityTests.cs

@@ -1,5 +1,4 @@
 using System.Text;
-using System.Text.Json;
 using NATS.Server.JetStream.Storage;
 
 namespace NATS.Server.Tests;
@@ -29,10 +28,14 @@ public class JetStreamFileStoreCompressionEncryptionParityTests
                 Encoding.UTF8.GetString(loaded.Payload.ToArray()).ShouldBe("payload");
             }
 
-            var firstLine = File.ReadLines(Path.Combine(dir, "messages.jsonl")).First();
-            var payloadBase64 = JsonDocument.Parse(firstLine).RootElement.GetProperty("PayloadBase64").GetString();
-            payloadBase64.ShouldNotBeNull();
-            var persisted = Convert.FromBase64String(payloadBase64!);
+            // Block-based storage: read the .blk file to verify FSV1 envelope.
+            var blkFiles = Directory.GetFiles(dir, "*.blk");
+            blkFiles.Length.ShouldBeGreaterThan(0);
+
+            // Read the first record from the block file and verify FSV1 magic in payload.
+            var blkBytes = File.ReadAllBytes(blkFiles[0]);
+            var record = MessageRecord.Decode(blkBytes.AsSpan(0, MessageRecord.MeasureRecord(blkBytes)));
+            var persisted = record.Payload.ToArray();
             persisted.Take(4).SequenceEqual("FSV1"u8.ToArray()).ShouldBeTrue();
 
             Should.Throw<InvalidDataException>(() =>