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>

src/wal.h

@@ -0,0 +1,65 @@
+#ifndef WAL_INCLUDED
+#define WAL_INCLUDED
+
+#include "metadata.h"
+#include "file_system.h"
+#include "config.h"
+
+typedef struct {
+    MetaOper oper;
+    uint32_t votes;       // transient, not persisted to disk
+    int      view_number;
+    uint64_t client_id;
+    uint64_t request_id;
+} WALEntry;
+
+// On-disk representation of a WAL entry (excludes transient 'votes' field).
+typedef struct {
+    MetaOper oper;
+    int      view_number;
+    uint64_t client_id;
+    uint64_t request_id;
+    uint32_t checksum;    // FNV-1a over all preceding fields
+} WALEntryDisk;
+
+_Static_assert(NODE_LIMIT <= 32, "");
+
+typedef struct {
+    int       count;
+    int       capacity;
+    WALEntry *entries;
+    Handle    handle;     // file handle to the WAL (0 = memory-only)
+} WAL;
+
+// Initialize an empty, memory-only WAL (no file backing).
+void wal_init(WAL *wal);
+
+// Initialize a WAL from an on-disk file. Recovers valid entries,
+// discards partial/corrupted trailing entries.
+int  wal_init_from_file(WAL *wal, string file);
+
+// Initialize a WAL from network data (memory-only, no file).
+int  wal_init_from_network(WAL *wal, void *src, int num);
+
+void wal_free(WAL *wal);
+
+// Move ownership from src to dst. If dst is file-backed, the file
+// is NOT affected; use wal_replace for atomic file replacement.
+void wal_move(WAL *dst, WAL *src);
+
+// Append an entry. If the WAL is file-backed, writes to disk and
+// fsyncs before updating the in-memory buffer.
+int  wal_append(WAL *wal, WALEntry entry);
+
+// Truncate the WAL to new_count entries.
+int  wal_truncate(WAL *wal, int new_count);
+
+// Atomically replace the WAL file contents with the given entries.
+// Writes to a temporary file, fsyncs, then renames over the WAL.
+// Updates the in-memory buffer and reopens the file handle.
+int  wal_replace(WAL *wal, WALEntry *entries, int count);
+
+int       wal_entry_count(WAL *wal);
+WALEntry *wal_peek_entry(WAL *wal, int idx);
+
+#endif // WAL_INCLUDED