feat(ha-repl-hardening): add batch entries list to WalCatchupEntry schema (task 1.2)

Add self-referential entries @3 :List(WalCatchupEntry) field with
$C.maplistcount("n_entries"). Single-entry path unaffected — n_entries
defaults to 0, entries to NULL when not using batch mode.

.kiro/specs/ha-repl-hardening/tasks.md

@@ -17,7 +17,7 @@
   - _Requirements: 1.1, 1.3_
   - _Boundary: Wire Protocol (controller.capnp)_
 
-- [ ] 1.2 Extend `WalCatchupEntry` for batch support in controller.capnp
+- [x] 1.2 Extend `WalCatchupEntry` for batch support in controller.capnp
   - Add `entries :List(WalCatchupEntry)` field for multiple entries per message
   - Regenerate capnp bindings; verify compilation
   - Compilation succeeds; single-entry decode path still works

src/include/typemap.h

@@ -286,10 +286,12 @@ typedef struct {
     xce_data_t *payload;
 } snapshot_transfer_t;
 
-typedef struct {
-    uint64_t    lsn;
-    uint8_t     entry_type;
-    xce_data_t *payload;
+typedef struct wal_catchup_entry {
+    uint64_t                   lsn;
+    uint8_t                    entry_type;
+    xce_data_t                *payload;
+    int                        n_entries;   /* batch: number of sub-entries */
+    struct wal_catchup_entry **entries;     /* batch: array of sub-entry pointers */
 } wal_catchup_entry_t;
 
 /* ── Persistence Cap'n Proto mapped types (P0-9) ─────────────────────

src/schema/controller.capnp

@@ -838,6 +838,7 @@ struct WalCatchupEntry $C.mapname("wal_catchup_entry_t") {
   lsn       @0 :UInt64 $C.mapname("lsn");
   entryType @1 :UInt8  $C.mapname("entry_type");
   payload   @2 :Data   $C.mapname("payload");
+  entries   @3 :List(WalCatchupEntry) $C.mapname("entries") $C.maplistcount("n_entries");
 }
 
 struct CtrlHelloRequest $C.mapname("ctrl_hello_req_t") {