feat(ha-repl-hardening): add snapshot chunk fields to capnp schema (task 1.3)

Extend SnapshotTransfer struct from 2 to 6 fields:
- snapshotLsn (existing), chunkIndex, chunkOffset, totalSize,
  isLast, payload (existing)
No resumption fields per restart-on-reconnect policy.

src/include/typemap.h

@@ -282,8 +282,12 @@ typedef struct {
 } upgrade_config_ack_t;
 
 typedef struct {
-    uint64_t    snapshot_lsn;
-    xce_data_t *payload;
+    uint64_t    snapshot_lsn;   /* checkpoint LSN this snapshot corresponds to */
+    xce_data_t *payload;        /* chunk payload (64 KB max) */
+    uint32_t    chunk_index;    /* 0-based chunk number */
+    uint64_t    chunk_offset;   /* byte offset in the full snapshot */
+    uint64_t    total_size;     /* total snapshot size in bytes */
+    int         is_last;        /* true (non-zero) for the final chunk */
 } snapshot_transfer_t;
 
 typedef struct wal_catchup_entry {