feat(ha-repl-hardening): primary snapshot chunk sender (task 3.5a)

ha_send_snapshot_to_standby reads snapshot file, splits into 64KB
chunks, sends each via OPCode_snapshotTransfer with correct metadata
(snapshotLsn, chunkIndex, chunkOffset, totalSize, isLast). Payload
encoding deferred. Hook seam for unit testing.
2 tests: 128KB/2-chunks, small-file/1-chunk. Tasks 3.5b-3.5d remain.

src/controller/config_repl.c

@@ -1730,3 +1730,86 @@ int ha_stream_wal_to_standby(endpoint_t *ep, const char *wal_path,
          "] to standby", standby_lsn, committed_lsn);
     return 0;
 }
+
+/* ── Snapshot chunk transfer ───────────────────────────────────────────── */
+
+#ifdef XCE_TESTING
+int (*xce_snapshot_chunk_hook)(endpoint_t *ep, xce_request_t *req) = NULL;
+#endif
+
+#define SNAPSHOT_CHUNK_SIZE 65536
+
+int ha_send_snapshot_to_standby(endpoint_t *ep, const char *snap_dir,
+                                 uint64_t checkpoint_lsn, uint64_t total_size)
+{
+    if (!snap_dir)
+        return -EINVAL;
+
+    /* Find the snapshot file by naming convention */
+    char snap_path[4096];
+    snprintf(snap_path, sizeof(snap_path),
+             "%s/snap-%016" PRIx64 ".capnp", snap_dir, checkpoint_lsn);
+
+    FILE *f = fopen(snap_path, "rb");
+    if (!f)
+        return -errno;
+
+    uint8_t *buf = malloc(SNAPSHOT_CHUNK_SIZE);
+    if (!buf) {
+        fclose(f);
+        return -ENOMEM;
+    }
+
+    int rc = 0;
+    uint64_t offset = 0;
+    uint32_t chunk_index = 0;
+
+    /* Chunk the snapshot file and enqueue each chunk via xce_request().
+     * Payload encoding uses NULL for now — the user-defined Data type in capnp
+     * (xce_data_t tagged union) requires a proper table dispatch for non-null
+     * payloads. This is a known limitation shared with wal_catchup_entry_t
+     * payload; both will be resolved when the capnp schema is updated to use
+     * the built-in capnp Data (List(UInt8)) type. */
+    while (offset < total_size) {
+        size_t chunk_len = (total_size - offset > SNAPSHOT_CHUNK_SIZE)
+                               ? SNAPSHOT_CHUNK_SIZE
+                               : (size_t)(total_size - offset);
+        /* Consume the file bytes to advance offset — data not used yet */
+        if (fread(buf, 1, chunk_len, f) != chunk_len) {
+            rc = -EIO;
+            break;
+        }
+
+        int is_last = (offset + chunk_len >= total_size);
+
+        snapshot_transfer_t chunk;
+        memset(&chunk, 0, sizeof(chunk));
+        chunk.snapshot_lsn = checkpoint_lsn;
+        chunk.chunk_index  = chunk_index;
+        chunk.chunk_offset = offset;
+        chunk.total_size   = total_size;
+        chunk.is_last      = is_last;
+        chunk.payload      = NULL; /* encoding deferred */
+
+        xce_request_t req;
+        memset(&req, 0, sizeof(req));
+        req.opcode                 = OPCode_snapshotTransfer;
+        req.body_which             = Request_body_nomsg;
+        req.body.snapshottransfermsg = &chunk;
+
+#ifdef XCE_TESTING
+        if (xce_snapshot_chunk_hook)
+            xce_snapshot_chunk_hook(ep, &req);
+        else
+#endif
+        if (ep)
+            xce_request(ep, &req);
+
+        offset += chunk_len;
+        chunk_index++;
+    }
+
+    free(buf);
+    fclose(f);
+    return rc;
+}