Fix WAL correctness: persist votes, fix crash recovery and view changes

Several bugs found via deterministic simulation testing (500 seeds, 30s each):

WAL format: Store full WALEntry on disk (including votes and checksum)
instead of a separate WALEntryDisk. This eliminates the need to
reconstruct transient vote bits after restart, and adds wal_update_entry()
for persisting in-place vote modifications.

Crash recovery: Restore 3-case startup logic (empty=normal, corrupt=recovery,
clean=replay). Fix last_normal_view initialization on clean restart. Call
adopt_view() after replay to reinitialize leader vote bits that may be stale
if a crash interrupted wal_replace before the atomic rename.

View change ordering: Move wal_replace before persist_state in
process_begin_view so the on-disk log is always at least as recent as the
persisted metadata. Reset votes on view_change_log entries before wal_replace
in complete_view_change_and_become_primary and complete_recovery.

View adoption: Add adopt_view() helper called from process_prepare and
process_prepare_ok when a NORMAL node learns of a higher view. Updates
last_normal_view and initializes leader vote bits.

Invariant checker: Track node liveness across ticks (prev_alive[]) so
min_commit monotonicity is correctly relaxed for nodes that just restarted.

https://claude.ai/code/session_01X2b5VRntA7LM32kdN9tPAM

src/wal.h

@@ -7,20 +7,12 @@
 
 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;
+    uint32_t votes;
     int      view_number;
     uint64_t client_id;
     uint64_t request_id;
     uint32_t checksum;    // FNV-1a over all preceding fields
-} WALEntryDisk;
+} WALEntry;
 
 _Static_assert(NODE_LIMIT <= 32, "");
 
@@ -53,6 +45,9 @@ void wal_move(WAL *dst, WAL *src);
 // fsyncs before updating the in-memory buffer.
 int  wal_append(WAL *wal, WALEntry entry);
 
+// Write a modified in-memory entry back to disk (recomputes checksum).
+int  wal_update_entry(WAL *wal, int idx);
+
 // Truncate the WAL to new_count entries.
 int  wal_truncate(WAL *wal, int new_count);