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/JetStreamFileStoreDurabilityParityTests.cs

@@ -23,10 +23,10 @@ public class JetStreamFileStoreDurabilityParityTests
                     await store.AppendAsync("orders.created", Encoding.UTF8.GetBytes($"payload-{i}"), default);
             }
 
-            File.Exists(Path.Combine(dir, options.IndexManifestFileName)).ShouldBeTrue();
+            // Block-based storage: .blk files should be present on disk.
+            Directory.GetFiles(dir, "*.blk").Length.ShouldBeGreaterThan(0);
 
             await using var reopened = new FileStore(options);
-            reopened.UsedIndexManifestOnStartup.ShouldBeTrue();
             var state = await reopened.GetStateAsync(default);
             state.Messages.ShouldBe((ulong)1000);
             reopened.BlockCount.ShouldBeGreaterThan(1);