Add WAL persistence to VSR log

Port the Write-Ahead Log implementation from raft/ into the VSR
server. Log entries are now written to disk with FNV-1a checksums
and fsynced before updating in-memory state. On startup, the WAL
file is loaded and validated, replacing the boot marker for crash
detection. View changes and recovery use atomic rename (tmp.log ->
vsr.log) to replace the WAL safely.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -1,5 +1,5 @@
 toastyfs
-toastyfs_client
+toastyfs_random_client
 toastyfs_simulation
 *.gcno
 *.gcda
@@ -13,5 +13,6 @@ coverage_html/
 example
 example_large
 examples/example
+.claude/
 .cluster/
 vsr_boot_marker