test(apiserver): add integration tests for P0+P1 backend features

Covers ~30 integration tests across 8 sections from the
apiserver-backend-integration-test-plan:

- §1: Cluster config push (store population, merge semantics, zero-preserve)
- §2: Host control (OnHostUpdated merge, validateHost, preCheck, batch ops)
- §3: Resource value push (all 7 value types, nil/unknown discriminant, overwrite)
- §4: Heartbeat keep-alive (interval update, zero-preserve)
- §5: Cross-feature (config+heartbeat, config+host, worker+config field preservation)
- §6: RBAC (cluster_admin bypass, algorithm confusion, expired tokens, hostgroup bypass)
- §7: Store thread safety (concurrent config pushes, host control+config, resource values)
- §8: Input safety (hostname length, empty lists, malformed payloads)

All tests use CGO_ENABLED=0 with //go:build !cgo build tag.
§8.1 hostname-length skipped pending P1-2b implementation.

src/frontend/apiserver/backend/val_helpers.h

@@ -0,0 +1,19 @@
+#ifndef VAL_HELPERS_H
+#define VAL_HELPERS_H
+
+#include "xce/types.h"
+
+// Thin accessors for xce_val_t union fields.
+// CGO cannot access C union fields by name (they appear as [N]byte in Go),
+// so these helpers extract typed values without unsafe pointer arithmetic.
+
+static inline int      xce_val_which(xce_val_t *v)  { return v->which; }
+static inline int32_t  xce_val_i32(xce_val_t *v)    { return v->u.ivalue32; }
+static inline int64_t  xce_val_i64(xce_val_t *v)    { return v->u.ivalue64; }
+static inline uint32_t xce_val_u32(xce_val_t *v)    { return v->u.uvalue32; }
+static inline uint64_t xce_val_u64(xce_val_t *v)    { return v->u.uvalue64; }
+static inline double   xce_val_fval(xce_val_t *v)   { return v->u.fvalue; }
+static inline char*    xce_val_str(xce_val_t *v)    { return v->u.svalue; }
+static inline int      xce_val_bool(xce_val_t *v)   { return v->u.bvalue; }
+
+#endif /* VAL_HELPERS_H */