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/clib.go

@@ -11,6 +11,7 @@ package backend
 #include "helper.h"
 #include "idgen.h"
 #include "controller.capnp.h"
+#include "val_helpers.h"
 
 // backend_ctx_t wraps xcecli_ctx_t for use as the C handler's arg.
 typedef struct {
@@ -49,15 +50,7 @@ static void jobctrl_set_ckptoption(xce_jobctrl_t *ctrl, xce_ckpt_options_t *ckpt
     ctrl->u.ckptoption = ckpt;
 }
 
-// xce_val_t union accessors — CGO cannot access C union fields by name.
-static int      xce_val_which(xce_val_t *v)  { return v->which; }
-static int32_t  xce_val_i32(xce_val_t *v)    { return v->u.ivalue32; }
-static int64_t  xce_val_i64(xce_val_t *v)    { return v->u.ivalue64; }
-static uint32_t xce_val_u32(xce_val_t *v)    { return v->u.uvalue32; }
-static uint64_t xce_val_u64(xce_val_t *v)    { return v->u.uvalue64; }
-static double   xce_val_fval(xce_val_t *v)   { return v->u.fvalue; }
-static char*    xce_val_str(xce_val_t *v)    { return v->u.svalue; }
-static int      xce_val_bool(xce_val_t *v)   { return v->u.bvalue; }
+// xce_val_t union accessors moved to val_helpers.h (shared with convert.go).
 */
 import "C"